I have 

Xeams Community Edition Version 5.3, build 5863.

It's look a like installation kit is broken, some files missing like /share/CACHEDEV1_DATA/.qpkg/QXeams/Xeams/config/config/UserConfig.xml

Yesderday I install your version to QNAP  and everything works fine, today I need to reinstall everything and after than I got lot off problems. That's not normal if use same installation set!

I also test install other email server and they not work either. I know that google doing something under it's not the first time.

Why there is com.google... java parts?

2020-10-25 20:00:05,701 ERROR [localhost].[/] - Exception while dispatching incoming RPC call
com.google.gwt.user.client.rpc.SerializationException: Type 'com.synametrics.livemonitor.shared.LiveMonStatusHolder' was not assignable to 'com.google.gwt.user.client.rpc.IsSerializable' and did not have a custom field serializer.For security purposes, this type will not be serialized.: instance = com.synametrics.livemonitor.shared.LiveMonStatusHolder@397d61cb
at com.google.gwt.user.server.rpc.impl.ServerSerializationStreamWriter.serialize(ServerSerializationStreamWriter.java:667)

I fix than to add /etc/hosts

127.0.0.1 localhost REAL_HOSTNAME_HERE

UserConfig.xml is not part of the installation package. Instead, it gets created as you create users in Xeams. Moreover, you should never re-install Xeams unless you first backup your configuration.

The RPC error you're getting is not because of DNS. It is happening because:

1. Some Javascripts have been cached by the browser and do not match with the server's copy. Click F5 in the browser to see if that takes care of the problem.
2. Perform a manual update following instructions on https://www.xeams.com/manuallypatching.htm

Contact our support department via email (support@synametrics.com) if the problem persists.