@Spinal
what!??

Okay I solved the problem for me. Here how I did:

1. go in the messenger offline
2. add maemo extras-devel: http://wiki.maemo.org/Extras-devel
3. update Pidgin Protocols plugin for Conversation and Contacts to version 0.9
4. reboot the N900
5. check ICQ account. you have to disable SSL.
6. try = win (?)

you should disable extras-devel after updating Pidgin Protocols plugin for Conversation and Contacts (see wiki).
ICQ is working fine again on my N900.