There's no certificates there, it is just a shared network key which can't expire.
First of all, check your SD card, it may wear off and produce I/O errors, so network key is incorrectly read from/saved to config.
You also may try to reinclude the sensor, so it gets an actual network key (in case it "forgot" it for some reason).
I think that I have no problems with SD. At least I don't see errors of that kind in logs. Besides, two installations have failed by its side and both of them have about six months of running. That's why I thought of an expiration problem.
I want also to say that the problem is related to z-way server and not to sensors, because each installation has paired more than 4 sensors and all of them begin to produce the "wrong authentication" problem at same time.
Really, excluding and reincluding the sensors they begin to work, but this solution it isn't practice for us because installations are remotes.
I have managed to find where the key is stored. It is inside a file in /opt/z-way-server/config/zddx directory.
In one of the installations that give me the related problem I find:
The updateTime correspond to "sun 01 nov 2015 13:59:55 CET GMT+1:00" that is exactly the instant when I began losing all multisensors. If I had known, I would have saved.
Anyway, do you believe the interpretation is correct? And why can this have happened?