If I edit a template look object and change a field value, the first autosave when clicking outside (loosing focus) is successful, but if I edit another field and click outside again, the second autosave ends in an error, and then the site is unusable with 502 errors every time.

I found out that the preview GET of the template look object is responsible for this.

The saveDraftPreview method generates the code for the preview iframe without checking the object class, but we should exclude here class groups like setup (and maybe other, potentially with a class group exclusion list in ini).
We should also check this list in first place to display or not the preview button.