I had some trouble logging into the Desktop App for Linux. I kept on getting the error that my username or password is incorrect.
After some struggling, and resetting my password on spotify.com, I tried to log in to the client using my username instead of my e-mail, and it worked. It seems like the client doesn't recognize the e-mail associated with my account when I try to login.
One possible cause is that my e-mail contains the '+' character in it.
Since it was a bit frustrating, I wanted to report it in case anyone else ran into this problem.