Sometimes during some changes in wp-admin, any other than default language ie. /de/ for some short time is pointing to default language, it causes that cache is holding it and all users are redirected to default language until we clear cache. Do you know about any problems with WPML and using cache (object cache, advanced cache, nginx cache)
I'm sure that you will understand that it is very difficult to give a general answer. We don't have any known issues with caching systems from what I know, but it may be something related to some particular caching setting.
I suggest you try going this route - if you're using multiple caching mechanisms, try disabling one by one or altering some settings within. Also, when this issue occurs, try deleting caching from only some caching mechanisms (if possible) and checking if the issue is still there.
This could give you a clue on which caching mechanism (e.g. object caching) is responsible for this. If you figure this out, then we can try emulating this scenario on our setup, trying to debug and find if there is potentially a compatibility issue.