First of all I want to thank you all for responding.
I followed the link provided by Mai Ali but the result was the same.
I created a new storage group and moved the (current) mailbox to the new storage group.
I tried to restore a few items from the users mailbox.
First he asks for a tape called "Dinsdag-2011", after about 7 minutes he asks for a tape called "Thursday-2011", after a few minutes he asks for a tape called "Week-01-2011", all these tapes I inserted as requested but shortly after inserting the last tape he canceled the job with an error. (see below)
Job ended: woensdag 14 mei 2014 at 13:59:25
Completed status: Failed
Final error: 0xe000848c - Unable to attach to a resource. Make sure that all selected resources exist and are online, and then try again. If the server or resource no longer exists, remove it from the selection list. Edit the selection list properties, click the View Selection Details tab, and then remove the resource.
Final error category: Resource Errors
For additional information regarding this error refer to link V-79-57344-33932
Some extra info:
On weekdays we do an incremental backup and in the weekend we use a weektape for a full backup.
We have plenty of diskspace on our server.
When the restore starts I see the progressbar wich isn't filling all the time.
Operation: staging,
Bytes are counting up from 0 to 13,8GB,
Rate around 1456MB/sec.
The items I want to restore are far from 13,8 GB, at most 5MB.
Regards Marcel