This thread is resolved. Here is a description of the problem and solution.
Problem: I cannot register my website because I'm getting the "invalid key" message.
Solution: Please make sure that you have all WPML related plugins up to date. If so, please delete the site key and create a new one.
If with the new site key you still getting the error, please report the issue on the support forum immediately because it may be a problem on our server and we would need to investigate the issue as soon as possible.
Relevant Documentation: https://wpml.org/faq/install-wpml/
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