I'm trying to sign a PDF file using a scanned signature. When I drag it onto the document, I'm getting an error (see the screenshot). When I click ok, the strange popup disappears, along with my signature. The same happens with a drawn signature, as well as with a typed one.

I'm using Acrobat Reader, version up to date as of 22/08/2023, Windows 10. This is a problem specific to one PDF file (presumably).