Bug: Multicamera edit not longer possible after using essential graphic panel
You have to restart the Application to get the multi camera editing mode working again.
Travis Inghram commented
Yes, I have had this happen when adding subtitles to a video using graphic window. I had 4 camera multicam edit and wanted to add titles over certain areas for dialogue. I switch over to gfx pane do my thing, go back to editing timeline and then my project would not work right. Sometimes it would refuse to play. What I started doing was locking my timeline when I switched to gfx pane and sometimes that allowed me to continue editing.
What also helps is when using graphics pane make sure to switch out of multicam view. I had to go to composite view because the graphics pane does not resize correctly for multicam view, it acts as though its just one screen and puts the graphics over several videos, its very buggy.