Both XGMs were and are on manual control (but the remote cables into the turbo pump controllers remained plugged in as usual) = both XGMs are "out of the VAC loop". 

In XTD2, I saw around 13h that both turbos were off (0 Hz) and the pressure was at 10E-4 mbar !

The green light above the ON-button was flickering. 
I switched parameter 023 (Motor TMP) to ON and restarted both pumps. The green light above the ON-button 
 stops flickering and goes steady green. 
 
---going to XTD9 
 
In XTD9, I saw that the upstream turbo was off (0 Hz) and the pressure was in the lower 10E-6mbar. 
The downstream turbo in XTD9 was still operating. 
Parameter 023 to ON, and restart pump. 
 
---going to XTD2 
 
Error codes 6, 6, 98, 2, 37 on one pump, error codes 6,6,6 on the other (XTD2).
I unplugged the REMOTE cables in XTD2 and XTD9 to avoid further trouble as long as we are on "manual". 
This info was also sent by email to the involved experts.