OK then I will in detail decribe what I did, because I'm not quite sure what was working for me this time:
1) tried to catch login in screen before error pops up by closing net connection 3 times - no luck!
2) tried again the procedure from @setter (#4 this thread) after closing FB on PC - no luck!
3) still closing FB on PC I removed FB account on N9 and tried open fmobi - no luck!

4) still closed FB on PC, open the FB account on N9 (not establish share or chat and contacts) net connected when asked and quick started fmobi, closed net, and then I succeded in catching the FB login screen, keyed in credentials and push login, the error screen pops up for a second and then wupti I'm in and everything are working fine! (Established share and chat/contacts on FB account without problems)