[Building Sakai] Column Name showing users ids

[Natalia Stash]

Dear Sakai developers,

For this academic year we use Sakai version 2.8.0.
Sometimes a strange bug appears - in different places where a list of students is being displayed the column "Name" suddenly starts showing students ids instead of the students names. 
So as a result we have two columns "Name" and "User ID" both showing users ids. 
After restarting Sakai we can see the names again. 
Do you know why this problem occurs and how to solve it?

Best regards, 
Natalia Stash.