Jukka Salmi wrote:

What about using record locking on a file in the logdir (maybe on the
logfile itself)? I.e. making sure that all involved processes always
acquire an exclusive lock, then write the log and afterwards release
the lock?

Amanda already lock the 'log' file.
But why it is not working on your machine?

First step, find which locking amanda use.
What is the output of: amadmin vv version | grep LOCKING
Which filesystem is used, is it NFS mounted?
Which OS?

Jean-Louis