double slash in asset URLs

site: https://opalschool.org/

I'm seeing a lot of 403 errors on a site of mine, and I see javascript errors in google developer tools console that always show a double forward slash in different plugin urls:

Failed to load resource: the server responded with a status of 403 ()
https://opalschool.org/wp-content/plugins/wp-smush-pro//assets/shared-ui/wdev-ui.css?ver=1.0 Failed to load resource: the server responded with a status of 403 ()
load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils,plupload&ver=6c00de4……:9 JQMIGRATE: Migrate is installed, version 1.4.1
https://opalschool.org/wp-content/plugins/wp-smush-pro//assets/shared-ui/wdev-ui.js?ver=1.0 Failed to load resource: the server responded with a status of 403 ()
https://opalschool.org/wp-content/plugins/wp-smush-pro//assets/shared-ui/notice.css?ver=1.0 Failed to load resource: the server responded with a status of 403 ()

When I am viewing the admin page for Smush Pro I get the above errors, and the editing buttons are not available... this makes sense since the double slash is keeping some assets from loading. I've tested and removing the double slash before the assets/ folder reveals the files are there.

I see the same thing happen for some other plugins too (learndash), and I just installed WP Smush Pro on this site today. So I know the issue has been happening for a bit.

I have tried the following with no success:
- changing the .htaccess file
- re-saving permalinks
- turning off redirect related plugins
- searching the database for" //assets/" in links to try to change any that may have been written incorrectly but were lingering

I have a staging site copy of the live site that is a recent copy in the last few days, and it does not show the errors. I have tried comparing plugins on both sites and not found any causes. I may get further with this comparison, but haven't had that much time on it yet.

Any ideas welcome... Thanks!