Hi @Marcel-1650 ,
>But now I've noticed that when adding a new Node to an previously upgraded SQL Failover instance (over 2019 installer) I'm unable to move the database to that new Node.
Do you mean that you cannot add a new node to the upgraded SQL Server failover cluster?
Or you have successfully added a new node to the failover cluster, but it failed when you manual failover SQL Server to the new node?
Can you show specific error messages. You can check the data path(Advanced) in the properties of the SQL Server cluster instance in SQL Server configuration manager on the new node.
You may need to modify the startup parameters of the SQL Server cluster instance on the new node.