Error trying to edit a 3.0 rule

Trying to edit the actions of this quite simple rule...

And receive this error message...

The logs don't show any errors. Actually everything still works smoothly but I want to make some edits. This error started to show after I'd already edited the rule a little to separate out a switch off action into its own action because it seemed to be getting lost and not always activated in an action that requested multiple switches to turn off. Not sure if that corrupted the rule somehow.

Here are the rule settings in case they help.

Any options?
Rebuild in v4.0?