I just checked the link you provided, as well as, browsed random pages on your site in both German and Spanish but couldn't find any 404 error.
Can you record a screen to show what exactly is happening?
Also enable debugging on your site. See https://wpml.org/documentation/support/debugging-wpml/ for details. Once enabled, browse the WordPress admin pages (random), frontend website and the said sections/pages a few times. This will log the errors in the debug.log file which you can upload somewhere and share the download link for investigation.