Hi,
I encountered the same issue.
There seems to be an issues with the dependencies for synapse in bullseye-backports.
Synapse 1.57.1 needs matrix-common~=1.1.0, but got matrix-common==1.2.1
So the only (safe)thing to do is to wait until it has been resolved on the packaging side of things. Until then, no synapse on freedomboxes.