OWA: no items to be displayed

Hello,

I'm searching for hours what might be the problem. Untill now no avail...

All my users can access OWA. Now we have some users (managers) who can
access almost every mailbox. Locally with Outlook this works fine! But with
OWA for some users they get a screen where there is no navigation tree. The
get the screen that is normally on the right side of the navigation tree, and
there is displayed: "There are no items that can be displayed in this view".
Changing the view doesn't change anything.
BUT, when the manager types for example:
https://example.com/exchange/petra/inbox the mails are displayed, again
without the navigation tree on the left. How can I get this navigation tree
back?

Note: when Petra logs in herself, everything is normal.

Any help will be appreciated!
Carl

.