In my case clearing cookies and site data for home assitant domain in my browser solved the problem. It looks like it’s a front-end issue? 
I too had this issue after editing an automation. After undoing all of my changes and adding them back one-by-one, I discovered it was due to single quotes being used instead of double quotes around a template… Ugh!
For example, inclusion in a single automation of:
'{{ state_attr('sensor.dummy', 'text') }}'
borked all of my UI-created automations, while:
"{{ state_attr('sensor.dummy', 'text') }}"
works.
If only there was a way to catch such issues and replace quotes properly…