Hello, the thread is quite old, but I was facing the same problem, looking for a solution which is quite simple: CHECK WHETHER YOU ARE LOGGED IN OR NOT. I wasn't logged in, offline, then I did it and all shortcuts were back active. 

Best regards 

Marcin

Seems legit! <facepalm emoticon here> Looks like RWS is continuing the good old traditions of reliability.

Any newfound solution to this issue? Just occurred in 2019 SR2. 

Adrian Komiński 

If you have merged segments, this command is disabled. This is by design, due to the potential danger of destroying the underlying source file.

Are you sure it is disabled upon merging segments? I'm not sure I tried it in Trados Studio 2022, but in previous versions, it was not disabled... and the few times I tried it after merging something incorrectly, it indeed caused a fatal error.

That being said, the CTRL+Z issue is not typically related to merging: it just stops working quite randomly. In my experience, it seems to only happen when more than one project is open, and it gets solved by restarting the software and making sure to open only one project... which of course is just an annoying workaround, not an actual solution. 

Are you sure it is disabled upon merging segments? I'm not sure I tried it in Trados Studio 2022, but in previous versions, it was not disabled... and the few times I tried it after merging something incorrectly, it indeed caused a fatal error.

That being said, the CTRL+Z issue is not typically related to merging: it just stops working quite randomly. In my experience, it seems to only happen when more than one project is open, and it gets solved by restarting the software and making sure to open only one project... which of course is just an annoying workaround, not an actual solution. 

Louis LaBonté 

If you merge across paragraph breaks, Undo will definitely be suspended. But indeed, sometimes it stops for reasons beyond my understanding.