Hi Wolfgang,
Issues with WPML usually happen if the Custom Ajax Handler is enabled. It’s a great performance tuning option, but it may lead to such errors. Please check if it’s enabled, and if so, make sure to turn it off instead: https://i.imgur.com/IG2m6Zu.png
Otherwise I won’t be able to tell unless I’m able to see and debug the issue directly via FTP. The fact that the title and the content are okay tells me that some aspects might not been initialized correctly when requesting the post URL, or this might be a yet unknown issue specific to a combination of plugins.
Feel free to update your ticket information with temporary back-end and FTP details, and I will check the problem and try to find the cause and fix it if possible.