I have the same problem on a production site have you found out a solution
For me the solution was the server who set a new PHP version higher than the need so i just change the PHP version
I am facing the same problem over here, and it has been almost 3 days without any solution. I have tried everything and ran out of google terms for search.
Did you guys find the solution? I tried all the steps that you also did, still didnt get a shot.
I ran those commands and it helped me
php bin/magento setup:upgrade
php bin/magento cache:flush
php bin/magento setup:static-content:deploy