Maxim,

After 29 hours error re-appeared jus tonce, which is much less than before

I see a correlation on my monit system at this exact time :
apache traffic had a peak, which equals to a big download peak

I'm now thinking to nginx tweaks i have not done yet

I now enlarge
from 64m to
client_max_body_size 256m;

From 256k
client_body_buffer_size 512k;

add :
send_timeout 300s;

This one could be issue :
sendfile_max_chunk 512k;
I put it to 0

This is too large too
directio 50m;
I put it as :
directio 4m;