Why does Back Up Now work from Time Machine Preferences but not from the status bar?
I think I may have found a workaround with the tmutil command line tool. If I say tmutil startbackup I get this The backup disk is not available error message, however if I add the destination it seems to work.

First I determined the destination of my backups:

xavier@imac-xavier:~$ tmutil destinationinfo
====================================================
Name          : WD_Backup
Kind          : Network
URL           : afp://wd_backup@DeviceName.local/WD_Backup
Mount Point   : /Volumes/WD_Backup
ID            : 11326F32-D5BA-4FE8-83FB-E9CBD8F6FF2D


I then used this ID to launch the backups with:

tmutil startbackup -d 11326F32-D5BA-4FE8-83FB-E9CBD8F6FF2D


I added it to my crontab so it runs hourly:

xavier@imac-xavier:~$ crontab -e
42 * * * * startbackup -d 11326F32-D5BA-4FE8-83FB-E9CBD8F6FF2D