I can’t reproduce your error, what did you enter to get in this situation?
Under /var/lib/nethserver/db/proxypass you can find your entries and remove them.
My “cleaned” /var/lib/nethserver/db/proxypass file looks like this:
# DO NOT MODIFY THIS FILE.
# This file is automatically maintained by NethServer
# configuration software. Manually editing this file may put your
# system in an unknown state.
# updated: Wed Oct 4 15:03:40 2017
Don’t exactly know if that’s the correct way, but it worked…
But I am really interested in how you came into that situation.