Update from the customer: He only wants to run a one-time backup of that computer. My colleague, which takes care for the customer, found another backup software which can be run as a portable app from an usb-stick.... That should be enough in his opinion.
Thanks for your help so far!
Btw.: In the meantime I tried the application startup control from Kaspersky to prohibit the start of the Veeam.Endpoint.Tray.exe... that works, too
VMCE 7 / 8 / 9, VCP-DC 5 / 5.5 / 6, MCITP:SA