Hi, I can confirm this error occurs on multiple different operating systems (fedora, ubuntu, arch, osx, windows 7/8/10) and also in all browsers tested on each OS. The error has to be on spotify's end. Since I am a developer by myself i looked into the traffic caused by the webplayer. When you simply reload the page a request is triggered, willing to deregister the current session and start a new one. Usually that request should return a success or something alike, but it returns an error containing the message:
I threw the connection ID registered at session startup into a diff view with the one contained in the request willing to deregister the connection, and both are the same. Therefore the error seems to be in the api part answering the DELETE request of the following url: "https://api.spotify.com/v1/me/notifications/user". Hope this helps your developers to trace and fix the problem.
The same thing is still happening to me after 2 weeks. I have tried logging onto my account on another device and it didn't work and I tried clearing cache and every other idea you have suggested. This is weird because it is only happening with one of my accounts and not my other one. Please help me out. Thanks!
I usually use Vivaldi browser but I tried on Edge (after going into my settings and disconnected all my devices from Spotify), and even though it wasn't my computer and it was a different ip address, I had that **bleep** message about too many tabs.
But I found a fix, now I'm using the web version of DEEZER (apparently thats what Spotify wants us to do as they don't seem too eager to fix the problem).
Your issue must have solved by now.. If that is not the case, then open a new tab in your browser and type YOUR ACCOUNT PAGE - SPOTIFY. In ACCOUNT OVERVIEW click SIGN OUT EVERYWHERE, just like you did before. This time you take your handset, log onto your Spotify app and play a song. Finally, refresh your browser. You should now be able to log in without any problem.