I tell you what Maneschi Romain, Futify developer, told me 3 days ago about this issue we have:

"Thank to let me know about this issue, I was not aware.
It seems to be du to a change in the spotify login process.
As soon as I have a fix, I publish a new release."

So, we have to wait...

@Tilo_HH
I have the same problem. With me, it happened that I changed my password, so when I tried to start again with the new password, correctly typed, It shows the same message: "authentification failed".

It's frustrating, because is the only way playing spotify on my UT mobile phone, and I need spotify... but I can't find a solution