I just wasted an hour of my time with this exact issue. Rebooting client fixed it as well. Amazing. Thank you Cory.
For epic - bug seems to happen when you:
Add a CapsuleTraceForObjects (example) to your BP, but you don’t have a custom channel type yet.
Go to project settings and add a new channel object type
Go back to BP you can’t select your new channel type yet. So Compile.
Now select custom object type.
Broken.
Restarting client fixes it.