Nasty bug: I suddenly lose undo and history

A nasty old bug has reappeared with Cubase here (latest 10.5), see here:
https://www.steinberg.net/forums/viewtopic.php?t=107851

The bug is this:
When working on the track, I suddenly can’t click a track to open its editor, and I can’t undo. History contains events, but can’t be edited. I can click and edit older events than before the bug hits, but not undo anything I do to them.

This has happened before in Cubase 9, and has reappeared now with a project probably started in 9 (though not in this project). The project uses Play from EastWest, HALion and a few Cubase plugs, the guitar ampsim S-Gear and a handful of audio tracks.

If I close the project and reload, I get a “serious problem”
If I close Cubase and reload, undo and history work as normal, but the problem has occurred several times. And if this happened on, say, an orchestral project with gigabytes of samples that need to load, this is a productivity and creativity killer of sorts.

Has anyone else had this?

I’m on WIndows 10 and using Steinberg UR28m.