From what the concerning user told me,
the issue occured randomly and is not straightly reproducible. It happend
four or five times and (as far as i know) was not solved by simple refreshing
but by moving to another week and then returning back. Which navigation
control the user was using, he could not tell anymore.
But one information might be important: The issue occured only on a remote
location, i.e. the user was connected to the server via VPN (with a rather
slow internet connection). Might it be possible, there is a synchronization
problem?