DT does not even have an activate methode, I think. At least, there’s none to be seen in the scripting dictionary. Which would make it throwing an error when you call that method expected behaviour. Edit I was obviously (or not so obviously) wrong: activate is a command (not a method, as I assumed). Thanks for pointing that out, @pete31
Thanks all. Oddly enough restarting Script Debugger didn’t solve the problem; and running the activate script directly through osascript still caused the error. But a reboot did the trick.