[Решено] WPML doesn't work on subdomains

Background of the issue:
I am trying to set WPML to work on different domains per language. The default URL is скрытая ссылка, which works fine. The Czech translation should work at скрытая ссылка, where the content of the default URL is visible, indicating that the redirection works. However, the language switch is not functioning, and there are no translations on the page at the moment. WPML cannot save the subdomain for the language.

Symptoms:
When I try to save the settings for different domains per language, it returns 'Not valid'.

Questions:
Why can't WPML save the subdomain for the language?
How can I get the language switch to work on the subdomain?