When you upgraded did you do anything to any of the marketplace files in $OH_USERDATA/jsondb? There is a formatting change between OH 3 and OH 4 that requires these files to be regenerated on OH 4. I don’t know if the new upgrade tool that has been released in M3 handles this or not.
Stop OH, move any file with “marketplace” in the name out of the jsondb folder and any backup for those files in the jsondb/backup folder also. If you miss the backups OH will just move to use those instead.
I think only the org.openhab.marketplace.json needs to be deleted but you may as well be thorough. But if you are worried you can just start with that file and see what happens.