UBB.threads PHP Forum Software Community Forums UBB.threads 6 Version 6 Support Fatal error: Cannot re-assign $this

Hello, I have been running version 6.? fr some time now but I needed to change to another server and now I receive this error when I go to the site:
Fatal error: Cannot re-assign $this in ubbthreads/ubbt.inc.php on line 1777

any idea why this is? And more important what action I can take to solve it?

Thanks a lot!

Cheers,
Alex