Solved
I searched through the php files until I found: class.ticket.php. I found the function_close(). Where it was erroring out on, was duedate="NULL", I removed this and I am able to successfully close tags via the GUI. I believe this is an issue with MySQL, as it does not accept NULL values, after the table has been written to. When I edited the values in MySQL, it gave me the same error. If there is a reason that I need this in there, please let me know, otherwise, close this thread and call it solved. Thanks.