I have entered the user names of known users but get the message 'no users found.'
It's case sensitive (I know that's dumb).
I thought about that but had the same negative result with the correct entries, including my own user name.
Weird, it works for me on your username.
That is strange. I tried to copy a screen image of my negative result using my user name but this comment field wouldn't accept that.
Can you send a screenshot to firstname.lastname@example.org?
I will send several examples.