I am using NextCloud app to locally sync my data to my Android Smartphone. Pointing Cryptomator to the master-keyfile works, but when I want to open it, it tells me the vault is not existing. Accessing in the same way my KeePass file works (even the sync when doing some changes) - so the sync seems to work.
Please check that the Masterkey file is not stored in root on your phone. The vault has to be a separate folder, with the Masterkey file in it besides the first subfolder „d“
the Cryptomator folder is not at root level and the structure is as you have described.
But I just noticed that the NextCloud client has not fully synchronized the folder and crashes while doing so. Let’s see what happens once it is fully synchronized. Has anyone here in the community tried the same (local storage with NextCloud client)?