I forgot to mention that it was in /c/europe, sorry. Apparently only English is allowed there, but my default language was German. I can still find the post from the failed user actions. The problem is that I cannot interact with the failed post at all - with the exception of the circled (i) at the top, which leads to the error message (and now I see that I also could long-press the text and copy it). The row of buttons below the text (upvote/downvote, comment, bookmark, three-dot-menu) is missing.
Edit: now I copied the text and posted it again (this time making sure that it is set as English), and it worked.