Yes, there are already posts of the same issue.
see here: Vault unlocks and assigns drive letter, but later the drive cannot be opened - #13 by tWCrypto
or here: Autostart working but mounting of drives fails (without an error message)
please check if your dokany version is the latest. But there are reports also that updating dokany didnt help, so expect not too much.
please try if changing the current session configuration does help (see other posts).
Last but not least: maybe changing to webdav is an option for you.