Hi Amanda,
thanks for this one. This is actually a problem I had immediately after upgrading to ArcGIS 10.8.1 from 10.2 (and VS2012 to VS2019). It was solved by changing the ESRI.ArcGIS.AddIns.11.targets reference in the project properties to ESRI.ArcGIS.AddIns.targets. Now other very similar addins run fine but one remains stubborn...
But to make sure, yes, "Register for COM interop" is not ticked and the ESRI.ArcGIS.Deskop.AddIns assembly reference Copy Local property set to false.
I really appreciate your help! Any other ideas?
Axel