Please don't take this as me being rude, I am really just being completely honest and to the point here... Your choices really are thus:
1. Upgrade to UBB.Classic 6.7.3
2. Upgrade to UBB.Threads 7
3. Don't use the PHPAccelerator
In short, UBB.C is dead, it reached End of Life some time ago, no one is working on it... The lead developer for it isn't even with the company anymore... There isn't really much anyone can do about it not working because there is no one with the knowledge of the product that CC had.
It's also not recommended to run UBB.C 6.5 due to the security issues; I know a lot of webhosts who disable sites for refusing to upgrade when the owners KNOW of security issues within the software and just don't feel like upgrading.
Basically, people get hacked for running older, buggy, versions of software; hell, some probably have been and just haven't noticed... Some troll probably sniffed the (plaintext) passwords out of some old member files via a malicious php script that they snuck in through a security hole... Knowing how a lot of people think, the password may be used for other things (even in some cases ftp password for their site)... Dangerous things there, those (plaintext) passwords and buggy code...
