Some things to try.
Install/reinstall the latest ILM.
What does the ILM show you is the *actual* license is on the iLoc?
Synchronize that iLok.
Do a refresh Subscriptions as well (any warnings there?)
(it's easy to imagine 2020.x won't' run if you have an old license on the iLok and have not updated it. But that won't explain why old versions don't run.).
Beyond that contact Avid Support.
https://www.avid.com/learn-and-suppo...-music-support