When using the:
Configure OpenNMS -> Customize Event Configurations
webUI, it seems to demand an Alarm Type.
1) Navigate to Customize Event Configurations
2) In the dropdown, select "3Com.events.xml"
3) Edit the first event "a3ComFddiSMTHoldCondition"
4) Change the severity from "Warning" to "Minor"
The screen will show a "cannot save changes" pop-up.
If you close that and navigate up the page, you'll see the error beside Alarm-Type and an exception that "alarm-type" cannot be null. I can't find the whole exception in any log, so the screenshot will have to do.