I am aware that this issue has been posted before, but it persists after trying all the fixes for it.

I am using a VZW Galaxy S6 with Spotify installed. My last.fm integration was working perfectly unstill it suddenly started throwing the Username/password incorrect error. I triple-checked that my credentials were correct. I checked for trailing spaces. I hit "enter" in the password field with every keyboard I had. I entered fake credentials and then re-entered real credentials. I cleared my app cache. 

Not only that, I can't even get the last.fm integration to turn off—if I switch the toggle to off, it switches itself back on when I start playing music and of course it throws the error again.