Because Version 1.5.x introduced the new vault format version 7, which is not compatible with vault format version 6 which was used in Cryptomator versions 1.4.x.
If you are now asking yourself: why a new vault format: here’s the answer: Vault Format 7: Behind the scenes
To check which version your vault has, open the Masterkey file with an editor. There’s a line with the version. I assume that you have started the vault migration after installing Cryptomator 1.5.x.
Correct? If yes, was it performed completely or was it interrupted? Is there anything in the upgrade logfile (if there is any)?
Do you have files on demand functions running or is your vault completely offline available?
Any ERROR or WARN Messages I’m your Cryptomator log when you just start Cryptomator or try to open a vault?