Thank you for your very quick reply. I am sorry I should have show my command
options in a first post.

$ httrack -q <TARGET_URLs> -O <DEST_DIR> -* +*.css +*.js +*.c +*.cpp +*.h
+*.csv +*.exe +*.zip +*.tar +*.gz +*.rpm +<TARGET_FQDN>/* -*.<SOME_FQDN>/*
+www.<SOME_FQDN>/* +*.jpg +*.gif +*.png -A32768 -%c2 -c2 -T60 -R2 -J16384 -n
-%k -s1 -%l en -B -e 

And I noticed I did not use -z option. Thus, I'll try to reproduce a download
failure under -z and -R9.

Akira