Some users are receiving the following add-in error.
If they close out the error the files work just fine. I've searched all over including the add-ins enabled on our Word client and have found nothing...has anyone found a resolution to this error?
I'm sorry but this may just be a client-side issue. We often have issues with Office and have to either have to repair Office, re-connect the Plugin, or in more dire situations, recreate their user profile on their machine.
If this Plugin requires either a Service, Feature, or Solution on the SharePoint side, you may want to try to uninstall/retract/etc then redeploy. Patching also can help this type of issue, but not normally.
If you give some more information on how the Plugin is installed/deployed to the users, what it's requirements are (Server side and Client side), we may be able to give better advise.
I was Having this same issue. The issue is the current build of your SharePoint farm.
You need to upgrade to a more recent CU.
I would set the log level of the trace log to verbose and reproduce the error while running ULS Viewer.
Somewhere you'll find SharePoint telling Word that this mysterious feature is not supported. And with this message from the ULS log, the community will surely be able to help you better :)
I documented the process of searching the ULS log a while ago on my blog: https://engineerer.ch/2019/01/04/how-to-find-errors-in-the-sharepoint-uls-log/