As usual, there are great suggestions here that are worth checking first:
1) Have you clamped your outputs with inductive loads? Diodes work great.
2) Do you have the updated firmware?
3) Is there any chance your 24V power is being compromised by a large 24V load? Have you sized conductors and filtered your supplies?
To add my experience with the ML1400 and that darn 71H error: used dozens and dozens of these on smaller machines as 'remote IO' that would message between each other. Love them. But some machines would come up with this error in spite of ensuring the above three items were handled. Very frustrating.
Then I put a 1400 on a drag boat. Mounted it on DIN rail like all my industrial jobs. Oops: worst case vibration scenario you could imagine. Vibrates like crazy at various frequencies and slams hard on every launch. The 1400 went about 5 seconds in the very first 8 second pass and croaked with this error: 1000HP blown alcohol that can't shift or declutch: nice. My fellow racing buddies thought I was an idiot for controlling the boat with 'all that electronics'.
So, Steve, I did what you asked about. I removed the DIN rail and mounted the PLC directly to a small secondary plate just large enough for the PLC and modules. Then I attached that plate to the main plate with rubber vibration mounts so the PLC is mechanically isolated from the rest of the system. Now that PLC has gone hundreds of passes without the 71H error.
I also started doing the same trick on every machine that I designed: I screw the PLC to a plate and use rubber mounts to mount that plate to the backplane. And I have not seen any more failures of our machines, even though I was convinced the machines saw minimal vibration. Now, admittedly, this is purely heuristic: maybe I over-reacted, its not science as I would prefer, but it is engineering, I think. I could not measure or analyze the vibration on my machines. All I can do is attest to the fact that by vibration mounting the 1400 on its own plate, all our new machines quit coming up with this 71H error that chased me for way too long. YMMV.