update your system, this old-old net-snmp issue.. probably you use Centos 6 or something same old..
I already had the discovery.php -u as part of the update script.
And I’m on the latest version of Ubuntu 16.04
Why it manifested now I don’t know but once I found it, I fixed it with
sudo rm -rf /var/lib/snmp/mib_indexes/*
sudo rm -f /opt/observium/mibs/*/.index
sudo rm -f /opt/observium/*/.index
Interestingly enough, there were no .index files under /opt/observium but there were some in /var/lib/snmp/mib_indexes