I found two possible solutions, both provided by Microsoft specialists.
Here you go:
'Usually this error occurs when Outlook is running in Compatibility mode.
Go to the location,
In 32 bit Operating system,
C:\Program Files\Microsoft Office\Office 12\Outlook.exe
In 64 bit Operating system,
C:\Program Files (x86)\Microsoft Office\Office 12\Outlook.exe
Right click on Outlook.exe> Properties.
Click on Compatibility tab, uncheck the box ‘Run this program in compatibility mode’.
Click on Apply and click on ok.
Try to open Outlook and check.'
'This issue may also be caused by a corrupted Outlook profile. Please follow these steps to create a new profile to test:
1. Close Outlook.
2. Right click on the Microsoft Outlook icon on the Desktop and click Properties. Note: If you cannot find it, double-click the Mail icon in Control Panel.
3. Click the Show Profiles button.
4. Click Prompt for a profile to be used and click Apply.
5. Click Add and type "test" in the Profile name box.
Note: Please don't choose the same name of your original profile.
6. Choose Add a new e-mail account and click Next.
7. Choose your email type and type the necessary information.
8. Click Next and click Finish.
9. Click Finish to exit.
10. Start Outlook.
11. Choose the TEST profile in the profile list and click OK. Does the problem occur in the new profile?
NOTE: If the problem is resolved in the new profile, you will need to import your data from the original profile. Please refer to the following Knowledge Base article on how to import
Best of luck