Outlook 2010 - Cannot display the folder

Hello,

I have a problem opening my mailbox in Office 2010.
After I click in a few folders I receive the error:
Unable to Display the folder:
Microsoft Outlook cannot access the specified folder location. Your server administrator has limited the number of items you can open simultaneously.
Try closing messages you have opened or removing attachments and images from unsent messages you are composing.

This doesn't happen with Outlook 2003...

I must note that I have a mailbox of 4GB with a lot(!) of folders in it.

Any help would be appreciated!

With kind regards,
Rogier