Well, I have good news and bad news. I'm going to document what I found & did for the benefit of anyone who might have similar issues in the future and find this thread.
This was uncovered while digging into the whole card 42 vs card 98 thing. I discovered that if I pass card=42,42,42,42,42,42,42,42 to the bttv module, the logs are spammed with 'rc rc4: error -6' through 'rc rc7: error -6'! This seems to confirm that what iconnor indicated - that these messages are coming from the capture card driver - is accurate. It also strongly suggests that mikb's hunch was right and that these errors are connected to the capture cards. The thread on the arch linux forum I found also makes sense if you consider that the person there was working on a DVR system with video capture cards in it. (Not improbable that he'd have the wrong card type in his config.) Further, it suggested a solution...
Because the cards were being auto-detected as type 98 and the 4 monitors using card inputs that were being autodetected worked, I simply removed the /etc/modprobe.d/bttv.conf file and rebooted, thinking that it would automatically detect all 8 tuners as card #98. Unfortunately, ZoneMinder wouldn't work with this configuration - the monitors for /dev/video0 and /dev/video3 on both channels 0 and 1 showed green and capturing, but all the rest of them showed red. Even those that showed green & capturing didn't have a picture, so I moved the bttv.conf file back into /etc/modprobe.d & changed it to be card=98,98,98,98,98,98,98,98 (Even though the manual that came with these cards clearly says that it's supposed to be type 42 & that's what our old server ran with for nearly 20 years, I figured it was worth a try.) The 'rc rc[0-3]: error -6' errors disappeared, further confirming that using card type 42 for the bttv module was the source of these errors.
Unfortunately, ZoneMinder still didn't work, even after rebooting, vehemently swearing at it, and threatening the machine. (Don't laugh - the last 2 are viable troubleshooting techniques which work a surprising amount of the time.

) I changed the bttv configuration back to card type 42, added 4 more log suppression rules, and rebooted... and it still refused to work. Discovered that in one of the numerous reboots, the BIOS swapped the device nodes of the two drives - the SSD boot drive was now /dev/sdb & the HDD storage drive was /dev/sda. (References to the UUID in /etc/fstab obviously didn't work anymore after I re-created the new file system with a bigger inode area, so I had just changed it to reference /dev/sdb1 instead of the UUID. Never seen a system change device nodes like that before...) Looked up how to get the UUID of the file system (blkid /dev/sda1) and changed fstab to reference the UUID instead of the device node. Rebooted, corrected the typo in the UUID, rebooted again, and both file systems mounted properly. Unfortunately, ZoneMinder still wasn't working.
By this time, it was an hour after I was supposed to go home, so I just left and picked it up this morning. A fresh look at it helped - found that the zoneminder service just was not starting. Issuing 'service zoneminder start' as root failed with the following:
Code: Select all
Starting zoneminder.service - ZoneMinder CCTV recording and surveillance system...
FAT [Can't create missing temporary directory '/run/zm': Permission denied]
Feb 20 09:52:40 vss-new systemd[1]: zoneminder.service: Control process exited, code=exited, status=255/EXCEPTION
zoneminder.service: Failed with result 'exit-code'.
Looking in /run revealed that there was no zm directory, so I created this, changed ownership & group to www-data, and voila! ZoneMinder starts.
To test the card type being passed to the bttv module, I changed the values to 8 * 98 in the config file and rebooted... to find ZoneMinder wouldn't start again - same issue. Again found /run/zm didn't exist, so did the same thing again & it started; all monitors have a picture and the syslog is silent as far as rc errors go. Confirmed that ZoneMinder is no longer automatically creating this directory with another reboot without changing bttv module options.
My plan is to simply script creating / chowning /run/zm at boot, but I'm curious as to why anything I did would have had any impact at all on that. Also, I'm going to let the system run with card type 98 (as it's functional & doesn't generate the rc errors) and see if it crashes again. Based on what I've discovered so far, I'd wager that my initial assumption that the rc errors were somehow responsible for the machine crashing was incorrect... which means I'm back to square one in that regard. Going to finish configuring remote logging today in the hope that it'll reveal something. Finally, I'm going to script sending myself an e-mail notification that the server booted, set it to trigger on startup, and add some positive number to /proc/sys/kernel/panic so that the system will automatically reboot in the case of a kernel panic. (I'm not even certain that the freeze IS a kernel panic as most panics I've seen have debugging information on the screen, leave behind dump files, and have the num lock/caps lock/scroll lock lights on the keyboard all flashing together. None of these conditions happened with this past crash, which is why I'm questioning if it's a kernel panic or something else. Regardless, it's something else to do that might help the system 'self-heal' after a failure... I can add file system checking, database repair, and running zmaudit.pl later if needed.)
Thank you all for your input & advice. If anyone has thoughts on my plans, I'd be happy to hear them.
This message is made up of not less than 90% recycled electrons.