I had several cFP-RLY-425s that stopped working. I was using them to switch small 120V AC Coil relays (180ma inrush, 33ma hold). In Max, it looked like they were loosing communications with the processor (the power & status lights were still on, even though it wasn't communicating).
I installed RC snubbers on every AC realy in the panel, but was unable to find the cause. I changed both the backplane and the processor. I also isolated the 24V supply that runs the processor.
After additional testing (using MAX) I found that, sometimes, when the processor would come out of a reset or reboot, it just wasn't communicating with the relay card...even before I closed any relays anywhere in the system...so I figured it had nothing to do with my design or install.
I had to get the project up & running, so I redesigned it to use cFP-DO-401s and DC relays, and sent the cFP-RLY-425s back to NI for them to figure out the problem.
It could be something as simple as the module wasn't completely/perfectly seated. We ran into this problem with two different FP-RLY-420 modules. About once a month, all the relays on a particular module would switch ON for no reason. A firm push to fully seat the module solved the problem.