-com.stackstaging.com- what is it? Where did it come from?

I did this website for a friend, using Divi. It's been pretty much trouble free since 2016 with minor fixes here & there and a migration to 20i.com last winter. Sometime between August and now, all the images disappeared. And this is a photography site. I've gotten sloppy and updated things without always checking afterwards, leaving it to my friend to tell me if something went wrong. But I guess he's been busy as well.

I looked at the media library and the media files are still there, but the associated posts are all broken. I looked at the actual links to the media files, and every one of them has changed from something like:
I don't know where this change has come from.

The links to each gallery also appear to have been corrupted. I rolled back the media plugins and updated the plugins & themes, to no avail. Wordfence was complaining about an htaccess file so I deleted that, also no improvement. Scanned with Defender and it found no problems.

I've been able to edit an image (Nature) by hand, but having to replace everything this way is going to be very tedious.

If someone could point me in the right direction, I'd appreciate it. I've so far found no insight with searching on "Divi images missing" or "stackstaging.com".

Support access has been granted x 5 days.

I unfortunately have to be away tomorrow (10/19, PTZ), but will check back as I can.

Even if I have to edit each link by hand, it would be nice to know where the "com-stackstaging." came from.

Thank you for your assistance. I look forward to the favor of your reply.