Hey all, the issue seems to be due to an updated version of the SimplePie module which is included in WordPress. (The story is, WordPress upgraded the included version of SimplePie to version 1.8.0, which was released last year; the new version introduces some changes to the type declarations in the code which take advantage of object-oriented features in more recent releases of PHP; these changes are no doubt a good idea in the long run, but they unfortunately break backward compatibility with code that extends some of the SimplePie classes to customize its functionality, as WordPress and FeedWordPress both do.)
Anyway, the good news is, there is a relatively quick fix to the immediate issue; I have put out a point release this morning (2024.1119) that includes a patch to fix this error. The recommended course of action is to upgrade FWP to this version. Give it a download, take a look, and let me know if it rids you of your troubles, or if there’s anything that comes up or anything you need to check in on.