Logging in while the Auto sign-in and sign-out for users from all domains option is enabled, results in the following console error:
Refused to display 'http://test.site/?wpml_sso_iframe_hash=e8570fabced11cc6bb500bd7b2fdbead3999a78ee89fecaa988dfcd1a259d7e9' in a frame because it set 'X-Frame-Options' to 'sameorigin'.
When using the Multiple Domain per language option, you may find yourself being asked to log in every time you switch languages. This happens because the admin session is not carried to secondary languages.
For login/logout across multiple domains, WPML uses a technique that may be blocked by some servers if they set the X-Frame_Options to SAMEORIGIN.
Our development team is currently working on resolving this issue.