Hi, I’m also having this problem.
Updated WP and all plugins and themes.
Running on WP Engine.
If I use your solution with my home page and “?wpso=check” right after the slash, I just get taken to the home page.
It works on WP Engine staging, which is not cached. Live site also uses ssl, whereas staging site does not, but not sure if that matters.
Can you tell me what url paths need to be removed from their cache for this to work?
Are there known incompatibilities for this plugin to work on WP Engine hosting?
Also, sorry to be grammar police, but the error grammar is wrong. It should say “An error HAS occurred.” Not “is”. Of course don’t all-caps the “has” in that sentence.