The migration does only to some changes on the Masterkey file and creates a new file called „vault.Cryptomator“). It does not make changes to the content vault files.
As the 1.5.x version claims that it cannot open the vault due to version 999, it seems that the migration was done successfully.
Please installl the laustest Cryptomator and try to open your vault and report if that worked, or if not which error message is thrown.