Question CKEditor not showing content

This issue occurs on some instances/posts.
This particular post has nothing out of the ordinary that could indicate what is causing this issue:
www.warenwirtschaftssystem-forum.de/orga...g-der-verkaufspreise

The images attached show that there is some content in the hidden textarea, but it's not shown in the CKEditor's content area.
However, the "missing" content is displayed normally on the frontend.

Also attached is an image of the error found in the Console.

The site uses no optimization Extensions, it's running Joomla v3.9.26, Kunena is v5.2.4

Occur that problem, only when the post should be be edited?

Yes, it occurs only when someone tries to edit a post.
And, not all posts, but those that are like this one (containing text and multiple images).

Already tried to reinstall Kunena (refresh the codebase), but that didn't help either.

A new installation will not help. It's a known issue in the CKEditor, and we have already reported this to the CKEditor team. The error occur, if the message contain wrong bbcodes and so it's not possible to edit or quote this message. The editor recognizes already a space before a close bracket as wrong.
Yesterday we found a fix to that, and we will release this version coming soon.