I ran into a similar issue.
I believe it reared its head after moving about 20 very simple materials into a more appropriately named folder. (I believe I had some undocked windows open during the move.) The materials were being used by various skeletal and static meshes in a handful of blueprint classes (namely character, and pawn (one parent, 1 child)).
After having done that, the only things I can edit on any given component are events (On X button pressed, etc.). Nothing else can be selected/changed unless the static/skeletal mesh is already a component in a class AND is selected in the viewport. Recreating the blueprint class from scratch doesn’t fix it.
Edit: Window > Layout Reset fixed it for me.