You cannot directly upgrade from 4.1.18 to 4.2. There is a bug with the upgrade process.
We had errors with SASL being able to access the LDAP DB to authenticate users. It kept
coming up with invalid credentials for the service account.
Below is a workaround for this.

MAKE SURE YOU HAVE BACKUPS BEFORE UPGRADING!!

First, upgrade from 4.1.18 to 4.1.21, then upgrade to 4.2

That worked. There must have been some subtle changes to LDAP after 4.1.18....

To upgrade from v4.1...
# /etc/init.d/insightserver stop
# killall -9 slapd
# /etc/init.d/insightserver recover
# ps -ea |grep slapd
... check to make sure slapd isn't running then perform the upgrade...
# rpm -Uvh insightserver-4.1.21...rpm
# service insightserver start
...check to make sure it does start properly
# /etc/init.d/insightserver stop
# killall -9 slapd
#rpm -Uvh insightserver-4.2.1...rpm
# rpm -ivh insightwebclient...etc
# service insightserver start
....check it all works