I think I have found the problem.
The GROUP command and XOVER command are occasionally sent to the server in a single batch.
The server should respond to each command in the same order, but I suspect that isn't always the case and might depends on the server software.
That should explain why you have this problems and others don't (different providers = different server software)
In update v0.60, I've changed the way Usenapp communicates with the server and hopefully this has solved the problem.
v0.60 will be released this weekend.