Windows profile doesn't connect under Windows

[Stefan Xenon]

For several months I use Pidgin (now 2.10.1) successfully under Windows.
Recently I copied the profile folder ("purple") to my Ubuntu computer
which has Pidgin 2.10.0 and surprisingly I can't login to my XMPP
account anymore. Pidgin tells me 'not authorized' (Nicht autorisiert)
even though the very same account profile still works under Windows! I
verified the password and it's correct. Also in Pidgin I added a new
XMPP account with the same and verified credentials but still the same
problem. Any advice how to analyze this strange behaviour is highly
appreciated.