I added your note to my local test site (running kiwitrees 3.2.3, same as you) using raw GEDCOM edit – no problem at all.
I tried editing it as raw GEDCOM – no problem at all.
I tried a normal edit of the note – no problem at all.
I then repeated each of those steps on my live site (Our families) with the same result.
So my conclusion is that this is a server error. Hopefully your server techs will be able to pin-point the cause. My “best guess” would be security protection that needs “tweaking” slightly to allow whatever it doesn’t like about that note to get through.