I have an assembly in which I've been running structural simulations for several days. I added a new study and it won't let me pick a new direction for gravity. I went back to an existing study and can't re-pick the gravity direction there either. I've rebuilt, saved, closed, rebooted, and none of those changed the situation.
This is the second time I've seen the issue. The first time closing and reopening the file corrected it.
In the first attached image, the setup looks like it's going to be correct. The various gravity vectors I'm going to use in this simulation series are in the pyramidal sketch at the bottom. The right most like has been selected, the gravity vector preview is pointing that way - but the line is not highlighted. When I move to accept the selection, the line is unselected and the error in the second image pops up. I can do this over and over picking lines, edges, or faces.
I've not found this bug reported elsewhere, but I wouldn't be surprised if it's a more general interface issue that shows up in other contexts.