I can't recall OTTOMH what conditions roughly cause it.
But it's been discussed in-depth in those threads, & in the bugs reported.
Nokia claim it's easily fixed by deleting all the accts, doing a full reset, & then re-creating the accts.
But there's at least one user that apparently has done that, & it hasn't resolved the issue for them.
But most other folks I've seen do this, it at least goes away for a while, if not forever (or until now).

IIRC there's also a command line fix that's been mentioned previously, I can't recall it now.
May be in this thread, or I may not have added it yet, from other spots I've saved the link.
http://talk.maemo.org/showthread.php...92#post1104892