I got a new private message:
usernameX wrote:
»Hi usernameY, bla bla bla blaaa bla…«
…and click it. It goes to the old message-thread but the new message is not there…
Refresh of the message thread does not help?
Nope. 
Not even my own messages show up now… 
Could it have something to do with me starting to use “Email notifications for messages”?
We’ll check out this issue!
I can for now at least say that this is not the issue.
Fixed
Thank you!