I'm glad that you have been able to isolate the issue to one workstation, that could make the rest of the process of fixing this much easier.
The issue seems like it is being caused by an interruption of QuickBooks and the process that is used to communicate with Outlook, I suggest that you start by making sure that QuickBooks is up to date. It is possible that the software is still waiting for its turn to install an update, which could affect stability.
If that does not work for you, I recommend performing a clean install of QuickBooks on that workstation which will reinstall the link between QuickBooks and MS Office.
There are some brilliant minds among the members of this community, so be on the lookout for another suggestion.