I’m not quiet sure if this does help but maybe a log file brings some light into this: How do I enable debug mode on Android?
Spontaneously I would have said that the admins of 21Vianet must not have allowed the Cryptomator application to access the data, but then this window appears and not the error message that the username might be wrong:
Did you enter your username including domain i.e. not only foo but foo@bar.com?