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If you get this error on YouTube trying to:

• remove some or all videos from YouTube "Watch History"
• "Clear all watch history"
• "Pause watch history"

and you have Video DownloadHelper add-on ...

... this is the solution that works for me:

• disable Video DownloadHelper from Firefox add-ons:
  
• restart the browser
• try again clearing or pausing YouTube watch history

... and voilà:

Your Watch History was cleared.

Your watch History is now paused.