Sorry to hear about your problem MGT. Flexing is never a good thing when it comes to a tile installation.
The foam based products from Wedi, Noble Company, Laticrete and Schluter Systems are all products I avoid like the plague here in Vancouver. The most pro-active step you should take first is to dig up the receipts for the Wedi Shower Pan and all info you have on the install. This info surely will be needed when talking to the Wedi support people. Was the pan installed over plywood sub flooring? If so did you use a self curing modified thin-set?
Pictures of the install steps you took would be helpful as well.
Then reach out to Bastian at Wedi. He is their top dog and will track down a local rep (if there is one) for you to talk to. It might be that a larger repair is needed. Would it be safe to say that the problem is located near the center of the shower pan?
Bastian Lohmann - Wedi Corporation