FreedomBox was unable to perform its setup routines as a dependency for matrix-synapse package was only available in bookworm-backports. A fix available in next version of FreedomBox. A workaround for the problem is to run the following command:
sudo apt install -t bookworm-backports matrix-synapse python3-canonicaljson
For more information see this thread: Matrix Synapse no longer working, update error "matrix-synapse is kept back"