proxy activation failed: No global specified; cannot bind proxy

I'm getting the following error from wireplumber, both when starting from via systemd or from /etc/pipewire/pipewire.conf:

wireplumber[126604]: <WpObjectManager:0x55976c7d9f20> proxy activation failed: No global specified; cannot bind proxy

(The error doesn't seem to happen if I start pipewire in a terminal though.)

Any idea what could be causing this or how to debug it?

This is on Fedora 34, using stock pipewire (pipewire-0.3.31-2.1.fc34.x86_64) and wireplumber installed from https://copr.fedorainfracloud.org/coprs/whot/wireplumber/ (wireplumber-0.4.1-4.fc34.x86_64).