The latest update of ArcPro breaks a few things including AddIns. I have been using the incredibly useful CIM viewer but this needs to be replaced with one compatible with ArcPro 3.0
Now the development team behind this AddIn have made the changes and resupplied a new 3.0 compatible visual studio project for compiling locally but I don't have the version of studio required.
I have been on to the github site and posted an issue which seems to date been utterly ignored, so I'm hoping by posting here that someone from ESRI will give the team a prod and they respond to the issue...