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!

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.

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