The only thought I would have is that possibly a QuickBooks add-in for Outlook was installed along the way.
My thoughts on checking:
1) Close Outlook.
2) Right-click the Windows shortcut normally used to launch Outlook and choose "Run as administrator."
3) Once in Outlook, go to File | Options | Add-ins.
4) See if there are any add-ins related to QuickBooks. if so, disable them.
Now see if the problem persists.
Hope this feedback helps.