The problem: my syslog is full of these three lines repeating
gnome-session[3926]: Expression error: unknown function
gnome-session[3926]: Net: down $ ( bitrate ( net.down ) ) , up $ ( bitrate ( net.up ) )
gnome-session[3926]: ---------------^So bitrate() is unknown.
The message is caused by the System Load Indicator (indicator-multiload), and more specifically a non-default value of the indicator-expressions key in the de.mh21.indicator-multiload.general schema. The value I had somehow managed to set there was:
['', 'Mem $(size(mem.user))', 'CPU $(percent(cpu.inuse))', 'Net $(bitrate(net.down))/$(bitrate(net.up))', 'Swap $(size(swap.used))', 'Load $(decimals(load.avg,2))', 'Disk $(speed(disk.read))/$(speed(disk.write))']
The default value, as I read it from /usr/share/glib-2.0/schemas/de.mh21.indicator-multiload.gschema.xml just now, is
[ "", "CPU $(percent(cpu.inuse))", "Mem $(size(mem.user))", "Net $(speed(net.down))/$(speed(net.up))", "Swap $(size(swap.used))", "Load $(decimals(load.avg,2))", "Disk $(speed(disk.read))/$(speed(disk.write))" ]
So the correct function to replace bitrate() is speed().