We found one more problem that needs to be addressed. This one causes issues with the portal islands not rebuilding. For that reason we're putting out a 7.5.4.2 release. There are a couple options to fix this one if you're already running 7.5.4.x.

Grab the 7.5.4.2 download and extract the libs/ubbthreads.inc.php file from it and upload it into your libs directory.

You can also manually fix it by following the instructions in this post.

If you're still running 7.5.3 or earlier, then you can just follow the standard upgrade procedure.

You can discuss this announcement here.