It just happened to me too, my Timelines made on blueprints I created yesterday. I saved everything, closed the project used my pc for a while and went to sleep. Today when I started the project again the unreal and tried to play test it showed me errors and it said only the timelines created yesterday got corrupted. Their names were changed to “Add Timeline…” but attempting to create another Timeline and naming it to the original name makes them Both get named the same and another error appears where the Engine stated that there is 2 statements of the same Timeline. Also attempting to add or edit tracks on the new Timeline result in Engine crash and so do trying to copy/cut the corrupted Timeline.
Since I didn’t commited the last night changes the only fix to it was to delete the old Timelines and remake the new ones with a different name.