Right there with you except for the "enabled again" part. So frustrating.

This was a great clue for me; thank you. I have 15.1 installed on an external drive, thinking I was being clever to not risk my main hard drive. Well, there's a line in the new Apple Intelligence & Siri settings panel that says "Apple Intelligence is not available when starting your Mac from an external volume." Sigh.

Thanks a bunch. In my case, I opened a PNG asset in Preview, exported it as PDF and then did as suggested above in the Attributes inspector and this indeed fixed the issue. Whoo hoo!