I have the same (asteroid mini) in my car. But I don't have problems with login. I found out, that it's important, that the internet connectivity works, before you start spotify.
If this doesn't work for you, there sould be a login history in the spotify app. When you are on the login screen you can press the menu button and have a look in the login history. Then you can select a user and login. (worked for me, but also only when there is a internet connection available)
What i think is annoying, that there seems to be no other way to sync your files locally except the surfstick or bluetooth connection. It would be nice if you could take the files from the notebook cache and copy them on the parrot sd card. This works, but the asteroid doesn't recognize the files and says 0% downloaded =/
Had a Spotify upgrade in my car yesterday - All seems to be running better now with beautiful sound. My mercedes has a strange optical cable sound system which is not compat. with most things - The Parrott asteroid mini integrates seamlessly with all 5 speakers and gets 11/10 inspite of the software issues. The options with the asteroid are endless but the killer app is the spotify streaming over my unlimited 3 mobile data plan. BTW the weather is wonderful at the moment!!!!
Oh and BTW the Asteroid ( I'm not from Parrott if anyone wondered) recognises my new USB3 powered minidrive sat in the glovebox - the one terabyte drive works fast and seamlessly without any power issues. I have 750Gig of back up music on the drive for when Spotify goes down....