With 1.6.x, you upgraded the vault to a new format that is incompatible with 1.5.x. You can downgrade the vault to the previous format: [1.6.x] Downgrade to 1.5.x
However, I’m not sure if this will actually change anything. Before you downgrade, what volume type have you selected in the Cryptomator settings under “Virtual Drive”? If it’s at WebDAV, try out FUSE.