Been a while since I used 3.x - looks like they changed the settings around a little. After looking back on your error, I believe that Suhosin is your issue. suhosin.post.max_value_length
suhosin.request.max_value_length Increase these two values to 1 million, or ask your host to do so if you can't - This may seem high, but it's really not. Off the top of my head, this is what IPBoards 3.x usually wants to use anyway. That fixed it for me back in the day, IPBoards made an official announcement about it but it seems that they removed the announcement after the IPS4.0 update, so I can't link you to it anymore ;( well I changed the values in the php.ini but ipb is not reading it, I think that the hosting provider must do it