I'm not sure it's the validator, but it does seem to be failing because of some kind of cert / auth issue. It's connecting, it just doesn't seem to make it all the way.
XMPP SEND:: <stream:stream xmlns:stream="http://etherx.jabber.org/streams" xmlns="jabber:client" to="im.contactatonce.com" version="1.0" >
XMPP RECV:: <?xml version='1.0'?><stream:stream xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams' id='3688202177' from='im.mydomain.com' version='1.0' xml:lang='en'><stream:features><starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/><mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'><mechanism>DIGEST-MD5</mechanism><mechanism>PLAIN</mechanism></mechanisms><register xmlns='http://jabber.org/features/iq-register'/></stream:features>
[Mono] Loading reference 6 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/DotNetty.Handlers.dll asmctx DEFAULT, looking for DotNetty.Codecs, Version=0.6.0.0, Culture=neutral, PublicKeyToken=bc13ca065fa06c29
[Mono] Assembly Ref addref DotNetty.Handlers[0x7132cb6180] -> DotNetty.Codecs[0x7132d13f80]: 3
[Mono] Loading reference 5 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/LPA.Platforms.Connect.dll asmctx DEFAULT, looking for CustomMonoTlsHandler, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null
[Mono] Assembly Ref addref LPA.Platforms.Connect[0x712044ef80] -> CustomMonoTlsHandler[0x7120464080]: 2
[Mono] Loading reference 2 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/CustomMonoTlsHandler.dll asmctx DEFAULT, looking for DotNetty.Codecs, Version=0.6.0.0, Culture=neutral, PublicKeyToken=bc13ca065fa06c29
[Mono] Assembly Ref addref CustomMonoTlsHandler[0x7120464080] -> DotNetty.Codecs[0x7132d13f80]: 4
Loaded assembly: System.Security.Cryptography.X509Certificates.dll [External]
[Mono] Loading reference 3 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/DotNetty.Codecs.dll asmctx DEFAULT, looking for DotNetty.Buffers, Version=0.6.0.0, Culture=neutral, PublicKeyToken=bc13ca065fa06c29
[Mono] Assembly Ref addref DotNetty.Codecs[0x7132d13f80] -> DotNetty.Buffers[0x7132d13e80]: 5
[Mono] Loading reference 1 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/CustomMonoTlsHandler.dll asmctx DEFAULT, looking for DotNetty.Handlers, Version=0.6.0.0, Culture=neutral, PublicKeyToken=bc13ca065fa06c29
[Mono] Assembly Ref addref CustomMonoTlsHandler[0x7120464080] -> DotNetty.Handlers[0x7132cb6180]: 4
[Mono] Loading reference 3 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/CustomMonoTlsHandler.dll asmctx DEFAULT, looking for DotNetty.Common, Version=0.6.0.0, Culture=neutral, PublicKeyToken=bc13ca065fa06c29
[Mono] Assembly Ref addref CustomMonoTlsHandler[0x7120464080] -> DotNetty.Common[0x7132cb6080]: 6
[Mono] Loading reference 4 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/CustomMonoTlsHandler.dll asmctx DEFAULT, looking for DotNetty.Transport, Version=0.6.0.0, Culture=neutral, PublicKeyToken=bc13ca065fa06c29
[Mono] Assembly Ref addref CustomMonoTlsHandler[0x7120464080] -> DotNetty.Transport[0x7132cb6280]: 6
[Mono] Loading reference 5 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/CustomMonoTlsHandler.dll asmctx DEFAULT, looking for DotNetty.Buffers, Version=0.6.0.0, Culture=neutral, PublicKeyToken=bc13ca065fa06c29
[Mono] Assembly Ref addref CustomMonoTlsHandler[0x7120464080] -> DotNetty.Buffers[0x7132d13e80]: 6
[Mono] Loading reference 4 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/DotNetty.Handlers.dll asmctx DEFAULT, looking for System.Net.Primitives, Version=4.0.10.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
[Mono] Assembly Ref addref DotNetty.Handlers[0x7132cb6180] -> System.Net.Primitives[0x7119257280]: 4
[Mono] Loading reference 2 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/DotNetty.Handlers.dll asmctx DEFAULT, looking for System.Security.Cryptography.X509Certificates, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
[Mono] Image addref System.Security.Cryptography.X509Certificates[0x71164fb000] (asmctx DEFAULT) -> System.Security.Cryptography.X509Certificates.dll[0x711d4f6800]: 2
[Mono] Prepared to set up assembly 'System.Security.Cryptography.X509Certificates' (System.Security.Cryptography.X509Certificates.dll)
[Mono] Assembly System.Security.Cryptography.X509Certificates[0x71164fb000] added to domain RootDomain, ref_count=1
[Mono] AOT: image 'System.Security.Cryptography.X509Certificates.dll.so' not found: dlopen failed: library "System.Security.Cryptography.X509Certificates.dll.so" not found
[Mono] AOT: image '/Users/builder/jenkins/workspace/archive-mono/2019-06/android/release/sdks/out/android-arm64-v8a-release/lib/mono/aot-cache/arm64/System.Security.Cryptography.X509Certificates.dll.so' not found: (null)
[Mono] Config attempting to parse: 'System.Security.Cryptography.X509Certificates.dll.config'.
[Mono] Config attempting to parse: '/Users/builder/jenkins/workspace/archive-mono/2019-06/android/release/sdks/out/android-arm64-v8a-release/etc/mono/assemblies/System.Security.Cryptography.X509Certificates/System.Security.Cryptography.X509Certificates.config'.
[Mono] Assembly Ref addref DotNetty.Handlers[0x7132cb6180] -> System.Security.Cryptography.X509Certificates[0x71164fb000]: 2
[Mono] Loading reference 1 of System.Security.Cryptography.X509Certificates.dll asmctx DEFAULT, looking for System, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
[Mono] Assembly Ref addref System.Security.Cryptography.X509Certificates[0x71164fb000] -> System[0x7120464480]: 20
[Mono] Loading reference 0 of System.Security.Cryptography.X509Certificates.dll asmctx DEFAULT, looking for mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e
[Mono] Assembly Ref addref System.Security.Cryptography.X509Certificates[0x71164fb000] -> mscorlib[0x7132c8fd80]: 132
[Mono] Loading reference 3 of /storage/emulated/0/Android/data/com.cao.chat/files/.__override__/DotNetty.Handlers.dll asmctx DEFAULT, looking for System.Collections, Version=4.0.10.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
[Mono] Assembly Ref addref DotNetty.Handlers[0x7132cb6180] -> System.Collections[0x711ece3900]: 9
Resolved pending breakpoint at 'TlsHandler.cs:102,1' to void CustomMonoTlsHandler.TlsHandler.ExceptionCaught (DotNetty.Transport.Channels.IChannelHandlerContext context, System.Exception exception) [0x00001].
Resolved pending breakpoint at 'TlsHandler.cs:119,1' to bool CustomMonoTlsHandler.TlsHandler.IgnoreException (System.Exception t) [0x00001].
Resolved pending breakpoint at 'TlsHandler.cs:737,1' to void CustomMonoTlsHandler.TlsHandler.HandleFailure (System.Exception cause) [0x00001].
Resolved pending breakpoint at 'TlsHandler.cs:765,1' to void CustomMonoTlsHandler.TlsHandler.NotifyHandshakeFailure (System.Exception cause) [0x00001].
Resolved pending breakpoint at 'TlsHandler.cs:188,1' to void CustomMonoTlsHandler.TlsHandler.Decode (DotNetty.Transport.Channels.IChannelHandlerContext context, DotNetty.Buffers.IByteBuffer input, System.Collections.Generic.List<object> output) [0x00001].
Resolved pending breakpoint at 'TlsHandler.cs:128,1' to void CustomMonoTlsHandler.TlsHandler.HandleHandshakeCompleted (System.Threading.Tasks.Task task, object state) [0x00001].
Resolved pending breakpoint at 'TlsHandler.cs:582,1' to bool CustomMonoTlsHandler.TlsHandler.EnsureAuthenticated () [0x00072].
Resolved pending breakpoint at 'TlsHandler.cs:83,1' to void CustomMonoTlsHandler.TlsHandler.ChannelActive (DotNetty.Transport.Channels.IChannelHandlerContext context) [0x00001].
Resolved pending breakpoint at 'TlsHandler.cs:319,1' to void CustomMonoTlsHandler.TlsHandler.Unwrap (DotNetty.Transport.Channels.IChannelHandlerContext ctx, DotNetty.Buffers.IByteBuffer packet, int offset, int length, System.Collections.Generic.List<int> packetLengths, System.Collections.Generic.List<object> output) [0x00001].
Resolved pending breakpoint at 'TlsHandler.cs:471,1' to void CustomMonoTlsHandler.TlsHandler.UnwrapCompleted (System.Threading.Tasks.Task<int> task, object state) [0x00001].
Resolved pending breakpoint at 'TlsHandler.cs:586,1' to bool CustomMonoTlsHandler.TlsHandler.EnsureAuthenticated () [0x000b8].
Resolved pending breakpoint at 'TlsHandler.cs:589,1' to bool CustomMonoTlsHandler.TlsHandler.EnsureAuthenticated () [0x000bd].
[Mono] Loading reference 1 of Mono.Security.dll asmctx DEFAULT, looking for System, Version=2.0.5.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
[Mono] Unloading image System.dll [0x711ce3e000].
[Mono] Image addref System[0x71164fb300] (asmctx DEFAULT) -> System.dll[0x7120463800]: 5
[Mono] Config attempting to parse: 'System.dll.config'.
[Mono] Config attempting to parse: '/Users/builder/jenkins/workspace/archive-mono/2019-06/android/release/sdks/out/android-arm64-v8a-release/etc/mono/assemblies/System/System.config'.
[Mono] Assembly Ref addref Mono.Security[0x7120464500] -> System[0x7120464480]: 21
XMPP SEND:: <starttls xmlns="urn:ietf:params:xml:ns:xmpp-tls" />
XMPP RECV:: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
XMPP SEND:: <stream:stream xmlns:stream="http://etherx.jabber.org/streams" xmlns="jabber:client" to="im.mydomain.com" version="1.0" >