Unfortunately that PM topic has become corrupt. Normally the code will see the corruption and hide the PM, but it looks like I added that after 6.6.1. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />
My first suggestion would be to upgrade the board. If that fails to work, delete the file named in the error (Members/pm_topics/T/00004827.cgi).