The customer uses two mailboxes: management@ and info@.
management@ has full access to info@. Therefore, info@-mailbox is automatically displayed when logging in as management@.
We set caching rather low (3 months) via GPO to preserve disk space. Now, when management@ tries to display mails in info@ which are older than three months, Outlook says "No items matching your search criteria have been found. Currently all messages newer than three months are displayed."
On the other hand, if browsing its own mailbox (not another one via full access), the behaviour is as expected. Outlook says: "There are more elements in this folder on the server. Click here for more information about Microsoft Exchange." Clicking the link will display all messages on the server.
For the time being, I found two workarounds:
increase caching limit (not preferred, see above)
Withdraw full access and integrate info@ as a second mailbox within profile (not preferred, because this means manually configuring two or three mailboxes per user)