I'm trying to implement Custom Links on two sites (because the Matrix plugin isn't working either, but that's another story).

On moodle.org (LMS) and hq.moodle.com (Workplace) the same thing happens.

I'm trying to put a typical Matrix link in the form (eg https://matrix.to/#/#moodle-english:moodle.com)

but the form just reloads with an error, like this:

I've tried different browsers, switched off browser extensions etc. No luck.

It seems to be something to do with the link, because it saves fine when I use a normal URL like https://moodle.org/

However, now on the course page there is still no custom link showing.

You can look on https://hq.moodle.com/course/view.php?id=11 where I ended up making a custom URL for Matrix chat https://moodle.me/allhandschat just so it would save, but there is still no custom link in that course.