Upgrading from Core to Standard - Blank screen

I tried upgrading a 2.9.2 Core installation to 2.9.2 Standard by following the instructions here. I did take note of the warning not to run the Update Wizard, so at that point I simply deleted the installer folder as directed.

Now however I am just getting the old white screen. I tried adding $debug = 1/2 to the index/admin files, but it’s always just blank.

Triple checked that the config.php and database.php files are correct. Permissions as they should be.

Regular html files and individual php files (eg. phpinfo) parse properly.

I had rewrite rules in .htaccess but I commented them out and reverted to index.php as the index page in the config.

When I change the index/admin files to point to the backed up system folder instead of the new one it works fine.

I am using a few addons (Detect Mobile, Freeform and Wyvern). I have a sneaking suspicion they have something to do with it.