I get the same problem, it seems to be one of those random taint problems where the default UI tries to blame addons for literally everything.
I don't know a fix yet. If you want to pinpoint it, you can type '/console taintlog 1' while ingame and reload the UI after you see a message showing "Interface action blocked because of an AddOn", then check the taint.log file in World of Warcraft/Logs.
It may well be an addon interfering? Are you using any addons that remotely touch the watch frame or its buttons?
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