The "C1" in doit.log means httrack is already using the --cache=1 behavior.

Looks like changing that won't help. Sorry.

WHRoeder explained that "resume", "update", and "continue" are three different
things in httrack.
<http://forum.httrack.com/readmsg/29895/29888/index.html>

"""
Resume continues exactly where it left off, all the data in RAM.
Continue, has to reread the cache files to get where it left off.
Update, starts at the starting URL and asks the server has this file changed
since last time (last time is in the cache.)
"""

You said you tried "resume interrupted download" and "update existing
download".

Based on WHRoeder's description, it sounds like you want to continue.

Is there also an option for "continue download"? Did you try it?
Otherwise, the command line version has a --continue option that might work
for you.