This topic recommended deleting a specific plugin.
How to fix OJS 3.3.0-8 http 500 error - Software Support - PKP Community Forum (sfu.ca)
However, I had to delete all of them (from the database/PHP My Admin) to get it fixed.
But, pages are too bad now… too similar to mobile pages… with no visible icons either. I had to use some keyboard tricks to access the icons.