ERROR=1 means that the bash script is setting the environment variable "ERROR" to the value 1.
apachectl basically does:
$HTTPD $ARGV ERROR=$? exit $ERROR
The issue is that /opt/local/apache2/bin/httpd -k start
is exiting with an exit code of 1. You should figure out why that is. You said you ran sudo /opt/local/apache2/bin/httpd -k start -e Debug -E /dev/stdout
but saw no errors. What was the exit code?