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!
I am in the midst of migrating our CMS 11.14 to the latest CMS 12.
As I am familiarizing myself with the new Admin UI, I noticed that there are nothing list under Scheduled Jobs, despite numerout in our database. I can see them when looking in Config > Plug-in Manager > Scheduler Job Administration.
Any insight or help would be greatly appreciated!
-Kevin