... I would say to unmount the diode, just to be sure.
Again, make a mark or write down the orientation of the diode, so that it can be re-installed without any problem. Maybe it would be simpler to take a picture of it before removing, for reference. ... The top surface side, that is.
... Here is a link describing how to test a zener diode, out of circuit, using the ohm scale of the multimeter.:
zener diode test
... the diode end with the black band is negative (cathode).
... the diode end without the band is positive (anode).

Hi drc,
Thanks for the explanation. Difficult for me to go on further on the subject: my time is becoming short these days (works is getting priority), anyway my soldering iron is too big to work without risks on the small zener area; but by chance a colleague just proposed me this morning to get a look to the card, he is great in the domain. I'll keep you informed of the process. Thank you one ore time for your help.
Didier

 

... very good Didier. I am interested to see if that zener diode is defective or not. ... Whenever you get to it. ... Have a good day.
... drc

 

... very good Didier. I am interested to see if that zener diode is defective or not. ... Whenever you get to it. ... Have a good day.
... drc

OK, I'll do that as soon as I can, this evening my colleague (his nickname is Bobine, in French it means both "weird face" and "coil") suggests to check the double resistance R578.
Didier