You need to upgrade to the latest version. Versions prior to 6.7.2 suffer from a bug when running on PHP 5.
If you really don't want to upgrade, you can replace every instance of $this with $thisUBB and that will allow prior versions of the accelerator to run with PHP 5.
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Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.