My client is getting the "ADD-IN ERROR - Sorry, we had to restart because this add-in wasn't responding."
I've read other posts and thought that this was fixed. My client has around 3300 contact that I've imported from their ERP system.
If you restart the Outlook client, does the issue persist? Please also provide us a screenshot about that error message for further checking?
Meanwhile, update the Outlook version to the latest one to check it. If issue persists, please let us know the detailed Outlook version.