There are cases that the Outlook add-in does not intercept every error message. This can be because of a various reasons. One of them is that the error is not entirely handled or Outlook itself crashes. 


Display Outlook errors

Under environment variables (system) set 'VSTO_SUPPRESSDISPLAYALERTS' to '0', then restart Outlook.


Outlook errors in Windows Event Viewer

Open Event Viewer.

1) Click on 'application '

2) Click on 'filter current log'

3) Set 'Event Sources' to 'Outlook'

4) Click 'OK'


The list should be filtered now on Outlook messages. Any critical event may contain the error log.