Wanted to post another update to this in case anyone is experiencing similar issues. The issue was “fixed” the last time I posted, but resurfaced shortly after. The issue kept occurring while viewing any page on our site while I was logged in to wp-admin, regardless of browser, and could only be fixed with a cache clear until logging in again. I finally identified that it was a plugin conflict between WP SOL and Autoptimize. Both plugins were trying to minify and group my HTML, CSS and JS. Autoptimize was installed after WP SOL, and must have overridden the SOL minification settings, but when SOL was updated, perhaps it did the reverse.
Thank you for the quick and detailed response!
I did manage to solve the issue – What I had to do was disable all of the minification/group settings, delete the cache and minified CSS/JS, and then re-enable those settings (plus a browser cache clear). No other settings appeared to be affecting the error message as I went through.
