SOLVED!
I had the exact same error: “Your account is already being used to play on FiveM or RedM.”
I tried reinstalling FiveM/GTA V, formatting Windows, using another Rockstar account, unlinking accounts and deleting DigitalEntitlements, but nothing worked.
**The thing that finally fixed it was deleting ros_id.dat.
Go to:** %appdata%\CitizenFX and delete: ros_id.dat
Then restart FiveM. In my case, the error disappeared immediately and I was able to join servers again.
Hope this helps anyone else with the same issue!