Please, confirm the permission for plugins deactivation, and switching to the parent/default theme. As soon as we complete the testing we will enable it all back, You would better make the full backup of your site.
You can create a clone of the site and move it to the staging platform, thus we can investigate the issue there.
Devsite – also called a Staging platform where you can test issues, enable/disable features/plugins thus such changes will affect the main site. The below article will help you to create it: