Okay, it seems you have successfully downgraded to 3.6.1. Un-installing and re-installing (as @Cranky has suggested) shouldn’t be needed.
From what I’m reading, especially the:
Suggests that darktable is somehow running in the background. Have you added it to your autostart sometime?
What I would do from this state is:
- Restart your PC to make sure darktable isn’t running. (Restart is better than shutdown and power-on)
- Open the file explorer and enable all file extensions being shown by removing tge check-mark of “Hide extensions for known file types” (google this term to find dozens of descriptions on how to do this)
- Make a backup of your darktable config folder (e.g create a zip file with the current content)
- Rename (or at this point simply delete) the data.db and library.db files.
- Rename the
data.db-pre-3.8.0andlibrary.db-pre-3.8.0by clicking on one, press F2 and removing the-pre-3.8.0part of the filename. Windows will complain that the filetype is being changed, but that’s what we want. - Now open darktable and cross your fingers

Maybe your virus scanner interferes, so you could consider disabled it while your perform above steps.