Ok I found a temporary fix for this.
In vbulletin admincp -> plugins & products -> plugin manager. Then scroll down to the chatbox section and click edit on the row that says misc_start. Search for...
PHP Code:
if (!empty($vbulletin->GPC['ccb_newmessage'])
and replace with something like this...
PHP Code:
if (!empty($vbulletin->GPC['ccb_newmessage']) && strlen($vbulletin->GPC['ccb_newmessage'])<256
I wrote the number 256 as I'm not sure what's the variable name the coder uses for limitations. If someone knows, please let me know too.