I just experienced this myself. My users are reporting it intermittently when creating threads, making posts, or sending conversations. There's no entry in the error log. I have no idea where to begin. It does seem to be intermittent. My initial thought was that it had to do with character length, but it's disabled (set to 0). Edit: I am copy and pasting and at least one member reported this error when copying and pasting. Might that have something to do with it?