Folders disappearing in Outlook 2013

Since a week or two week I've problems with the navigation pane. I have had outlook 2013 for about 9 months now and I didn't have any problems till now.

Folders disappear from the navigation pane. Sometimes all folders disappear, sometimes only some folders disappear. For example only the Inbox is shown or only Sent items and Deleted items. This happens in the favorites section too. I put for example inbox, sent items and deleted items in the favorites section but after some time they will all be removed from the section.

It's very weird. When I startup outlook often all folders are shown in the first second but the next second randomly some folders might disappear. It does not happen at every startup. Therefore restarting outlook often helps, except for the favorites section. Once a folder has disappeared from the favorites section it doesn't show up again. So I have to do the recovery of the favorites section myself.

(The configured account type is POP.)