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Welcome to WPML support forum.
While one of my teammates takes care of your case I will provide you with the first debugging steps.
We are currently having issues with a recent option that was added to WPML 3.9.4. It is the "Keep wpml.org up-to-date about which theme and plugins I use" option right below the registration key field. If you uncheck it you will be able to register the site key as usual.