[Resolved] Translation strings of website content missing

Background of the issue:
I am trying to translate strings of German custom content using the WPML string translation menu. This method is the easiest and fastest way for me and my coworkers to translate new content on the website. I have been experiencing issues with the translation editor presenting different versions to different users, loading poorly, and saving unreliably. Since some days/weeks, possibly since the last update of WP/plugins, the content strings are not showing up anymore. I tried rescanning via 'WPML > Localization of Themes and Plugins', but it only led to a huge list of strings from all my plugins, including WPML itself and some standard texts. The custom content I generated is missing. Strings that have already been translated still show up for users on the front end. I checked common problems like memory limit, which is set at 256/512MB, but that didn’t help. Link to a page where the issue can be seen: hidden link

Symptoms:
Strings of German custom content are not showing up in the WPML string translation menu. The translation editor presents different versions to different users, loads poorly, and saves unreliably. Rescanning via 'WPML > Localization of Themes and Plugins' only shows a list of strings from all plugins, not the custom content.

Questions:
Why are my custom content strings not showing up in the WPML string translation menu?
Could the recent update of WP/plugins be causing this issue?