Update | Jan 23, 2019
Microsoft has fixed this issue with version 16.2 (their December 2018 release). 

Although the issue is resolved we will retain this article. Details of the issue are below.

Microsoft is working to fix a bug affecting all add-ins that cause them to stop working if a user clicks off of Excel.

More information on the Microsoft bug is available here.

The issue is reproducible on the following versions of Excel for Mac

Meanwhile, the workaround is to ensure that Excel remains the app in focus, ie you should not click off Excel to any other app while operations are in process. If you do (or must) click off Excel the app will suspend operations and resume them when you return.

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