Problem Description
When upgrading Nagios XI above version 5.6.5, BPI checks which were dependent on hostgroups or servicegroups would change from their usual status to "Unknown BPI Group Index"..
Resolution
The fix for this issue involves editing affected checks in the Core Config Manager. Modify affected hosts and services by going to Configure -> Core Config Manager, and clicking "Hosts" or "Services", then clicking the "Edit" button (wrench icon) to the right of the affected host or service.
You will need to edit the host or service and prepend the $ARG1$ value with hg_ (if it's a hostgroup) or prepend it with sg_ (if it's a servicegroup).
After editing, click the "Run Check Command" button to verify that the new command works. After the change has been applied to all affected hosts and services, go to Quick Tools -> Apply Configuration to finalize your changes.