I had these problems since about 1 1/2 month or so. I've written twice to Spotify, who so far has neglected my messages.

Offline:

Most of the time when using the app, I am forced into an offline-mode. It is not enabled in settings. It does not matter wether I'm on mobile connection or WiFi.

Syncing:

I found out that if I completely uninstall Spotify and all it's files, I will be online when I log in the first time. I can then sync songs ONCE. After that, I am forced into offline-mode again. If I occasionaly go online, Spotify won't sync any list at all, or worse...

...erase my music:

Spotify claims that I have synced three devices, and then just erase all my offline songs. The thing is that I have only synced one device - my phone. This is of course VERY FRUSTRATING!

I use a Samsung Galaxy S2 with Android v2.3.3.