Changing default cover works but throwns an error that makes it seem as though it doesn't

Changing the default book cover redirects to this URL with a "No input file specified" error, but successfully changes the cover anyways. The error simply makes it seem as though it didn't work right. I don't recall this happening in the past so I'm guessing some recent-ish codebase change has made this start happening.

Same bug as reported here: https://www.librarything.com/topic/365316

Closing as a duplicate.