In my case I had to remove all rows from the table tblPlugIn. With that said, I thought that Episerver would've solved that specific issue with this fix: https://world.optimizely.com/documentation/Release-Notes/ReleaseNote/?releaseNoteId=CMS-22398

Anyway, run something like

SELECT * 
INTO tmpTblPlugIn
FROM tblPlugIn

DELETE FROM tblPlugIn

... and see if it works. Make sure you re-run the application so that you don't run into caching issues.

Edit: I did have the exact same issue, and if clearing tblPlugIn isn't enough - you could also check if you have any no longer existing scheduled jobs in tblScheduledItem. If a scheduled job is in the database but not in code, that could be a potential issue.

I had the same issue, and my solution was (as Andreas') to remove a couple of lines from tblPlugin.

Thanks Andreas and Tomas. 

Despite my prior conclusion that your suggestion was not effective, I discovered that I was in fact editing in the wrong database! 😱

So I have edited this response to indicate success!