I did some more tests related to the Jabber file transfer.
The problem was that on our local network transferring files with sip communicator was a problem - Pidgin and Psi couldnt send files to Sip Communicator ("Peer doesnt have file transfer capabilities" error message or the transfer was notified on Sip Comm and after clicking "Accept" it just hang there, in the XMPP console there was an error 404), sending files between 2 Sip Communicator clients was limited to 39 KiB/s (this is on a gigabit LAN) and sometimes it failed (error 404 in the terminal).
In the same setup Pidgin/Psi had no problem in transferring files at maximum speed (up to 30-40 MiB/s between clients gigabit LAN connection) both ways, never failing.
Now i had a laptop with Windows XP on it and tested it too - turns out that from Windows to Linux (Sip Communicator - Sip Communicator) the file transfer worked at max speed (9 MiB/s, 100 Mbit LAN). From Linux to Windows it was again limited to 39 KiB/s.
Sometimes the transfer (Linux -> Windows) failed either silently (no visible messages, the receiver clicked accept and nothing happened - in the terminal a 404 (not found?) message was displayed) or after a few seconds it failed - this time the message is displayed in the chat window.
In our network there are a few vlans that are used, but removing them from the Linux box (leaving only eth0) did not change the situation. Both test machines are plugged in the same gigabit switch (that in turn is connected to the main switch), using the same subnet and gateway.
I may assume that this problem is limited somehow to Linux? Because the Windows version can transfer files full speed.
OS used : Ubuntu(9.04 and 9.10) and Debian Squeeze all with the same results (Java version didnt matter - 1.6.0_16 through _20 or the bundled one all produced the same results).