Advantest R3751A VNA "Synthe Unlock"

[Marco_Mar]

Hi, my name is Marco from Italy.
I've been using Advantest R3751A VNA since two days ago when a sad message popped up from the screen: "Synthe Unlock".
Not immediately though: as soon as I power on the unit, it works fine for a few seconds, then the quoted message pops up.

The Service Manual of this instrument is nowhere to be found.

Any idea?

Cheers