For which message, with what browser/OS? I could type Test <Shift+Enter> Documentation and successfully save it in my private wiki (Firefox 102.6.0esr on Debian 11).

The detailed error message is needed to investigate this.

Line feeds are not prohibited in message documentation.

@Nikerabbit - What I wrote in my original comment was the complete error message. It was just “Error saving message documentation”.

@Tacsipacsi - I could not find which message it was. I’ll try to reproduce the problem with a different segment.

BTW I’ve partially reverted the title of this ticket because it was reworded incorrectly. This is about the “message documentation” (annotation subpages, /qqq; “message documentation” is how this is labelled in the UI), not about translated messages (target strings). Since I’m still trying to reproduce it the title might still be wrong but this is at least more accurate.

I'm really not sure that the line feeds are a problem. They are allowed in message documentation, and they are often used in it.

A couple of comments about that:

• You don't have to do it with Shift-Enter. Just Enter is enough. As far as I can see, the effect is the same.
• Like elsewhere in wikitext, to actually get paragraph separation, you need to type two line breaks (press Enter twice).

I've just tried saving a documentation message with line breaks on translatewiki.net, and it worked. I use Firefox on Ubuntu with English US keyboard. Perhaps other browsers, operating systems, or keyboard layouts do something unusual, but in this bug report, I don't see any more clues as to what the problem is.