Lots of threads about people upgrading to Premium, say on a PC, then being unable to log into their phone? For me the problem was that I'd set up the account on my phone, using my EMAIL address; then accessed the upgrade link 5 mins later on a PC, again using my EMAIL address. Then unable to get back into Spotify via the phone, despite the email and password being identical to what I was entering on the PC which was still clearly working.
Solution is that you need to use USERNAME when logging back in on phone, not Email address, even though I'd not created or used the username before. I found that suggestion buried somewhere in problem pages but it wasn't easy to find and clearly many others are frustrated too.