Ivan,
I have heard of the Calendar error, but we haven't been able to recreate it on any of our development systems, and the error is buried deep within JomSocial. I can only recommend contacting them to see if they can at least determine where the conflict is.. as we haven't been able to whatsoever. It's very rare, and seems to only occur on sites with a lot of modifications to either JFBConnect and/or JomSocial, which makes it all the more difficult to diagnose.
One user did say that they disabled a conflicting plugin, but after asking for further clarification, we never found out what plugin it was. You may simply want to disable (likely system) plugins to see if it goes away. If so, that will definitely help narrow things down for us.
Good luck, and if you find out anything, please let us know!
Alex