This looks to be a plugin/addin conflict. Revit 25 & 26 have a different runtime.
Lets look at GH plugins first, in a new instance of Revit open Rhino and start Grasshopper with GrasshopperLoadOneByOne and say no to 3rd party gh plugins.
A quick test in Revit is to start a new instance with no other Revit addins by Shift+Cntl on the Rhino start button.