For me this error curiously appears when in the following scenario:
Using a CameraActor, parented to a CameraRig_Rail, parented to an empty Actor object, all in a folder lol.
Just the way I set up an orbit rig, but maybe I need to change for future reference. Only way I got it to save was by converting camera in sequencer from spawnable (for rendering) back to possessable, so, annoying but easy enough fix.