That appears to be somewhat of a bug. The value of the Uid in the cookie is being set to "deleted" instead of actually being deleted.
What you can do to fix this is edit the libs/ubbthreads.inc.php script. Line 2056 looks like this:'
if (!$Uid) $Uid = 1;
Change that, to this:
if (!$Uid || $Uid == "deleted") $Uid = 1;