This seems fixed, as I can build this package again using the sbuild program from bullseye (in a bookworm chroot).
Because the package itself has not changed, I assume this is due to a change in the dependencies of one of its build-dependencies. If somebody knows for sure which package it was, feel free to reassign and add affects src:libsoup2.4. (Maybe it was php-xmlrpc, so I'm Cc:ing Ondřej) If not, I don't think it's worth the effort to investigate (the list of build-dependencies is too long). In either case, thanks a lot to whoever fixed this.
