REVIT
There is a known issue that is caused by Revit 2021 application when using V-Ray for Revit.
When opening a project that has never been saved with V-Ray for Revit, or when starting a project from scratch, enabling V-Ray and then opening another project with an existing V-Ray storage (i.e. V-Ray settings) leads to the following error message:
There are two workarounds to counter the problem.
Solutions 1
Save the first project with V-Ray and then open the other one.
Solution 2
Change the order of opening the projects and open the one that has V-Ray settings first.