THIS IS WRONG. 1.14.15 can unlock vaults of format 6. I’m really sorry for this mistake. Editet my post above.
To avoid confusion here: Vaults created with a Cryptomator version below 1.5.x need to be migrated. The migration itself can be done with Cryptomator 1.5.x (we integrated a user workflow for this). But the vault needs to be intact.
The first error you posted might have a solution. For more info, read: