Hi,
if you can't attach anything in RoundCube client then I suspect it's a problem with server configuration. RoundCube is a separate application used by us and we don't change anything in it's code (except addition of several plugins). So if you can't add any file then maybe it's some server issue. Maybe you ran out of disk space where temporary files are located? Try to find any server logs, restart apache or find out was there any software updates on your server.
Regards,
Adam