At one user's account one folder cannot be displayed. Roundcube shows "Server Error (OK)".
The problem occurs at folder Sent, and is still there in another folder when manually moving the messaging to its cur folder. So it seems that some messages are corrupted, but Thunderbirds opens folder and messages without problems.
There is nothing logged into error log. IMAP and MySQL log do not show anything interesting.
Any hint is appreciated.
I had the same problem... I managed to fix it, but i've forgotten how i did it!
Double check your config settings and disable your plugins and see what happens.
The problem does not occur in the current nightly build. So it seems to be a bug in RoundCube.