[Geschlossen] 404 page

Background of the issue:
I am trying to resolve an issue where 404 pages appear on my website when the WPML plugin is enabled. This does not happen when the plugin is disabled. The issue can be seen on this page: versteckter Link

Symptoms:
404 pages appear on the website when the WPML plugin is enabled.

Questions:
Why do 404 pages appear when WPML is enabled?
How can I fix the 404 error caused by WPML?