Thanks a lot for your immediate help. After applying patch MixMonitor
started works normally,
I can understand that this can be Happen in asterisk 10.4 but as a stable
and Long support version 18.104.22.168 this should not happen. I got same error
in both version.
Anyways this patch solved my problem.
44 bytes is very interesting, as this is the canonical length of a .wav
header. It sounds as though something is crashing immediately after the
recording starts, leaving no data.