Thanks for your reply Brian. For me a restart doesn't help. I haven't tried uninstalling the app yet. Also waiting doesn't help. Weeks without addition of the credits have gone by. :-(
I've posted the question to WGT, who pointed me to a third party. I guess I'll just have to accept that it's (very) hard to get credits without pulling the old credit card. It's a shame, I really like the game.