Confirm 1.4.15 is the Right Version To Open an Older Vault Backup (04/18/20)

Yes, 1.4.15 is the most recent version to open vaults of vault format version 6 (i.e. pre 1.5.0 update). The vault format version can be found inside the `masterjey.cryptomator" file of the vault.

This might be also the reason, why the migration fails. The migration does not fix an already broken vault. If you still have a backup of the old vault, you can use the sanitizer tool to fix problems: