I am using the excellent AirSane (sadly unpackaged for Debian):
https://github.com/SimulPiscator/AirSane

In order to get it to work, I had to increase MaxConnections to 2 for the
scanbm.socket systemd unit file.

I am sorry, it took me about 8 hours of work to get scanbd functional (I
will be filing other bug reports that I hope are helpful), so I did not
investigate this issue further to work out exactly what the problem is; but
raising MaxConnections by 1 seems a fairly simple and low-risk workaround,
if indeed there is a bug somewhere here.