Ok so I eventually found out how to do it, but still it fails. I opened deemix-gui, opened firefox, went to 127.0.0.1:6595, logged in with the deezer account and pasted the spotify playlist link in the search bar. Exactly the same errors appeared and there is no option to select songs from the spotify playlist at all. (I think this option comes up if I use a deezer playlist instead.)
May I ask
Have you tried this today to check if it still works?