Odd.. I tried troubleshooting this today and it seems that if I remove the Editor component (and just leave the Inspector fields), I can add the block okay.
With the Editor intact, if I remove all of the components from within it so it just returns an empty div, I can add the block to the page. Here’s where it gets funky though. With the empty div block added to the page, I began re-instating each component one by one to see where it breaks down, and I was able to add them all back in. However, with the components intact, I can’t add a NEW block to the page (I get the error above), but the existing one works.
Then I tried adding a new Form Fields block each time with one less component on it. It seemed that the ToggleField component was the culprit, as I got the block to add successfully, but then subsequent tests to confirm that failed. Not sure if something’s cached there or not, but it might be something to look into.