When starting to write the name – the username I want to send to shows up.
But as soon as I either use my arrow-down button, or hover over the username with the mouse, the list of usernames just disappears.
I thought it was due to the PM-plugin update, so I uninstalled it and then reinstalled the old. Now this doesn’t work either. Same problem.
And all the PM’s are gone! 
This sounds like a js conflict… Any reported in console?
Wp 3.8 with sp 5.4 and latest plugins?
Don’t really know. How can I check if it’s a javascript error?
yes, all updated!
open the browser console… and look for reported errors there…
if you want to pass us a link to your site, we can check for obvious ones, but of course, wont be able to pm as guest…
and what about the version question?
Link: http://goo.gl/W6X9qA
As I already said: Yes, they are all updated to the latest version.
While I can not say for certain that this is the cause there IS an issue with your site that sees the jQuery.UI library being loaded twice – the second copy coming from the actual jQuery site instead of from your WordPress site server.
This will cause problems here and there but whether it is responsible for your current problem I can not tell. The issue is described here:
http://codex.simple-press.com/codex/faq/troubleshooting/what-is-this-jquery-conflict/
and if you can find out just what is loading it up then we can most usually find a way to fix it up.
Thank you for having a look!
Can the double jQuery come from one of my plugins? Any idea on where I should start to look?
I can’t see why it would be necessary to import jQuery twice?
From another plugin OR your WP theme. There is no way for us to tell from the generated HTML.
It is NOT necessary to load a second copy and whatever is doing it is stomping all over the WP API and doing things totally incorrectly…