I signed up using my Google account and now my username is a number which I cannot change to a more meaningful name although it seems to accept it. Every time I press "Save Changes" a message "Saved!" appears but if I log in again, it is the initial number-username again.
Do you sign in on Duolingo by connecting from your Google account rather than by putting your log in details on Duo's site? I'm sorry if I've misunderstood your post, I'm trying to think if the username has been tied up to a process for some reason. It could be that it takes some time to come into effect though. Changing your username should not be a problem, unless your new one has already been taken up by another Duolingo user.
Yes, exactly that. I thought that it took the profile's id maybe, instead of my profile name, because I had not created a profile in Google+ yet. I set up my profile then, to see if that was the problem and nothing changed. The name seems fine, not too common, and Duolingo accepted it ok. Still my profile name is a number though. :/
Yes, in your profile page I see both the number for the Username and the name for your Duolingo profile description. The trouble is that that particular username has already been taken up as a username by someone else. The description can still be the same, it doesn't matter.
I suggest you try another username. Don't just change what letters are in capitals, it won't work unfortunately. I hope you have a username you like soon! :)