You may receive an error message when sending PMs at the moment. The message you're trying to send has been sent and if you refresh your screen, you will see it. Otherwise you may get many repeated messages. We're working on this!
I know, I use spotify everyday, both on my computer and my iPhone. I'm a little annoyed that there is close to no Japanese music on spotify though. But it's possible to upload local files, so I'm still satisfied!