v5 is about 4 years old and has a number of security problems. In addition, Infopop is no longer supporting any 5.x or earlier after March 15.

The requirements for v6 can be found at https://www.ubbcentral.com/ubbclassic/requirements.php

With regards to your specific problem, I'll have to defer to someone who has worked with v5 more than I have <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />