Just recently this popped up:
It gives no clue as to which light, automation etc.
I can find effect: None ten times in my scenes.yaml…
Usually repairs are really helpful, but this one is vague. Do I just have to replace all instances of ‘None’ with ‘off’?
Yes, but just use the Find and Replace All option if your YAML editor has that feature.
Unfortunately, Repairs is not going to assume states.
Thanks. 
Yes, my editor does, I use the File Editor add-on. I’ll get these edited and see what happens!
A little more info
Thanks for the info. Unfortunately I ran into a problem. I changed all instances of effect: None with effect: off but for some reason, the scenes still don’t work, and when editing them I see an error:
However, I have solved the issue by editing each scene, deleting the lights in the scene and adding them again - presumably this corrected whatever YAML had been broken over the many updates since they were created. It took a while as I had to go through and set all color and brightness values again which was a bit of a PITA, but at least I only had 10 offending scenes. It was absolutely necessary to delete each light, as it was un-editable in whatever state the YAML was in.
My mind boggles how this has been allowed to happen to the config over time though? It’s taken me over an hour to manually sort this, and the repair system in HA basically just said “you got until October to sort this” lol 
Thanks for your help @LiQuid_cOOled, I’m going to mark the git link as the answer because this is certainly half of the solution with the possibility that the other half is recreating/editing some scenes.
Yeah, unfortunately not ideal!! Your post will help others understand more!
In HA 2025.5.2, the logs contained the light name that was using effect: None which certainly helped me to track it down. I then edited scenes.yaml and replaced all occurrences of effect: None with effect: 'off'. Notice the quoted 'off', this was critical, otherwise you get the ‘Bad request’ error noticed by @robbie21. Then reload the scenes from the developer tools options.
Only updating this because it took a bit of digging to get all the pieces of the puzzle together and avoid having to recreate all the scenes.
Seems like here is my time to shine 
I had the same issue and Google did not find anything. So I tried around.
Turns-out, the scene I used had a list of effects - none of them applied but only the ones that the light supports.
And this was is - just edited from first effect “none” to off, error gone.
I had to do this for any light in the scene, but it is a really quick fix.
Might be a bit weird to wrap your head around the fact that it is not an applied effect at all in this moment in time. This was also actually the thing that kept me searching for much too long.
See here:
