New "Selected font failed during last operation" error, not fixed by usual solutions

My Photoshop has suddenly developed a fault where clicking the text tool on any new document gives the error "Selected font failed during last operation. If problem persists please disable the font." It doesn't matter which font is selected.

Weirdly, I can still add and edit text on some older documents, but not all.

Adobe Support spent several hours trying to fix this on my PC, with various versions of Photoshop, so they've tried all the existing solutions such as doing a full uninstall, removing the font cache files and folders, etc, but nothing made it work. I've tried Windows 10 Safe Mode and the problem is the same, and I've uninstalled recent Windows Updates too.

I installed the trial version of Illustrator, and trying to do anything with the text tool in that program caused it to hang every time for about 30 seconds. I haven't had problems with any other programs, and my PC is otherwise behaving perfectly. I've also tried signing into Photoshop with a different account (error remained) and signing into Photoshop on a different PC with my usual account (no error on the other PC).

I'm running Windows 10 Pro 20H2 on a PC with an Intel Core i5 4690S 3.2GHz CPU, 32GB RAM and an Nvidia GeForce GTX 1650 1GB GPU.

I'm a professional IT support specialist so I know how to troubleshoot, but unfortunately it seems the only sure-fire fix is to reinstall my OS, which I don't really want to do at this stage. What I need to know is what actually triggers the "Selected font failed during last operation" error, so that I can try and figure out why this is happening.

Thanks
Cliff