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This thread is resolved. Here is a description of the problem and solution.
I can't register WPML. It tells me "invalid site key".
The client generated a new site key for "www.sitename.com" while his website's link was "https://www.sitename.com" ("https://" was missing) so just creating a new key with the correct complete link made things work.
Relevant Documentation: https://wpml.org/faq/install-wpml/#register