I've had 2 fail in the last two weeks.
Two different customers have had the same thing happen.
Advanced monitoring says the card is working.
Little check mark.
The card itself seems to reboot with it's watchdog over and over. The card flashes once, tries to load and then after a few seconds this starts over.
On the second system, it was 5.1, upgraded it to 6.1 and that didn't help. Replaced the card, and the replacement card immediately starts working.
Each was about 5 years old.
I'm used to rarely having the problem actually be the card as opposed to telco.