Hi Steven,
>> however we have noticed that when Outlook 2013 starts.
Does this problem happen to all Outlook 2013 clients? or only one specific client? How about other version of Outlook client like Outlook 2016\2019?
>> It seems to be related to addins, but we can't be sure.
To check whether this issue is related to addins, you can start your Outlook in safe mode (Press Win + R, type “outlook /safe”, press Enter.) and see if the issue continues.
If it works fine in safe mode, then the problem is likely with one of your add-ins. You'll need to disable all add-ins after restarting Outlook in normal mode, and then enable them one by one to figure out the problematic one.
In addition, I found this article may be useful to you:
Using Multi Factor Auth in Outlook 2013.
Hope this can be helpful.
Regards,
Aidan Wang
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