Just to update with resolution. It turned out not to be the throw-out bearing. After pulling the clutch and not seeing anything obvious with the throw-out bearing, I tried turning the input shaft to the transmission by hand. With two fingers turning it I could feel a form of grinding every revolution for a fraction of a full turn. So, I pulled the transmission apart and expected to find something obvious wrong, but with the gear stack on the bench everything spun perfectly

Closer inspection did reveal 3 of the little teeth on a dog ring broken off - I found them in the casing. However, that would not explain any grinding. Uwe helped me replace the dog ring, and I test assembled the transmission spinning and testing everything as I gradually put it together. It turned out that when the front casing of the transaxle was put on the grinding would reappear. We had a spare front casing part that I put on, and that solved it

I could not see anything wrong with bearings in the casing, but that must be the issue, as it is now running perfectly again (with the spare front casing part).
Jes