As mentioned (but unreplied) in this thread:
This originally appeared with my MyCloud Mirror on FW 1.x but is still present in 2.10.302 as follows:
If the username of the admin user is not admin (in fact I selected my first name during initial setup) the authentication to Dropbox does not work. It just returns to the dashboard as if the authentication was not yet started.
Only if I change my username to “admin” in the dashboard usermanagement and try again, it will work flawless.
This bug should be easily reproducible so I hope it gets solved with the next firmware if possible.