I think that I found where the problem occurred…
I have a major axis on which I generated a cross-section. Later, I defined a couple of new axes that I needed to apply the additional load (ex. load from the safety barrier, cornice, etc)
When I delete these additional axes, the error disappears and I can access the Cross Member Editor. Maybe this is not the right way to define these additional loads?
I could not recreate this error message on my computer.
Are you working with the latest service packs?
If the error can not be solved with an software update, could you please send me the project files?
Perhaps I can recreate the error within your project.