When running list-packages in emacs on an up-to-date Debian Buster (10.6) I get the following error:
error in process sentinel: Error retrieving: https://stable.melpa.org/packages/archive-contents "incomprehensible buffer"
The exact version of the emacs package is 1:26.1+1-3.2+deb10u1. When using wget on that URL and looking at the downloaded file it looks like a valid e-lisp data structure to me.
This has been working in the past and works fine in the MacPorts emacs package version 27.1_5.