API Error Troubleshooting

Every time I try to change the spells on my sheet I get the same API error.

I did my research on the forum and internet before but every solution hasn't led to a change. I've cleared my cookies multiple times as well as my browser cache, logged in and out, disabled all of my extensions, tried different browsers, tried it on mobile and PC, waited a week and tried again, disabled and re enabled home brew, I even downloaded the app. I still get that API error and can't do anything on the sheet! Any tips would be desperately appreciated.

Check for homebrew spells and content. Remove them.

One sneaky culprit may be homebrew where somebody added a subclass into the "available for classes" box for the homebrew spell, which shouldn't ever be done because it means anyone in the campaign using that subclass has the spell forcibly added to their sheet, since homebrew is automatically shared with everyone in the same campaign.