Thanks alot ....
I found out what's the problem... and solved it... 
I'm using the SQL server version of the forum, and this problem is not there in the Access version...
The problem is because of the datatype of the field stroing the message body, which is set to 'TEXT' and it should be 'NTEXT'... once I changed it, it's working fine with no problem...
I noticed that you are using the 'NVARCHAR' in all fields, and in the signature field you are already uisng the 'NTEXT' type... only in the message body and the private message body it's 'TEXT'...
I think you need to fix this in the distribution version...
Thanks again...