Certain emails can't be marked as "read". Why?

I again and again have emails that remain highlighted as unread even though I opened and looked at them. Even explicitly selecting it and pressing the [Unread/Read] button or right-clicking the email and selecting "Mark as Read" does not change it's appearance and thus it also remains in the "Unread"-selection.

I can delete such emails but sometimes that's not what one wants. I might want to keep the email but get it into the "read"-state. 
I haven't found any commonality between these emails - it appears to be random that they are "frozen" as "unread".

Why is that so? And how can I force such emails into "read"-status?