Class Extensions Fail at 10.4.1

[HeatherMorrison]

Hello Folks,

We have a number of VB.NET class extensions that we use to present users with attribute forms when editing data. We are in the process of upgrading to 10.4 and have already upgraded our code to use .NET Framework 4.6.1 and 10.4 ArcObjects. The class extensions seem to be applying correctly using the same method (AlterClassExtensionCLSID) but when we try to save changes ArcMap crashes. Has anyone else experienced this? At this point our upgrade from 9.3 to 10.2 is starting to look smoother... help!

This is the problem signature we get when ArcMap crashes. Always the same dll...