That's what I got after half a day of thinking the hardware was broken, 10 unsucessfull starts and some wired fixing:
opnsense: /usr/local/etc/rc.bootup: The command '/usr/local/bin/rrdtool update /var/db/rrd/opt2-packets.rrd N:U:U:U:U:U:U:U:U' returned exit code '1', the output was 'ERROR: /var/db/rrd/opt2-packets.rrd: illegal attempt to u...
I think your file system crashed and corrupted during an unscheduled power-off. It's best to reset that particular RRD data file from reporting: settings. Also do a health audit from system: firmware to ensure no system files have been damaged with it.
Cheers,
Franco