I recently had this issue where we were suddenly unable to use an add-in that we have had on a network folder for years with no problems. We made certain changes to the way we map our user's desktops/profiles, involving Sharepoint directories. Copying the add-in file to the Add-In folder on each users machine works for us! However, it only appears in ArcMap and not in Catalog for some reason. Windows 7, ArcGIS Desktop 10.5
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