As it turns out, I was able to narrow the problem down to having 0ad installed and needing to be upgraded.
I determined this by manually upgrading everything (running `guix system reconfigure /etc/config.scm didn't change anything, nor did force-restarting guix-daemon using `herd`). I'm not sure what the specific issue is - increasing the verbosity level didn't actually give any more output - so I just uninstalled that package.
For my part, I think this can be closed (I'm not sure how to do that) but the 0ad package seems to be the one that has that issue.