Hey Elmar,
Here is the description for the BFFA4020 error.
(Hex 0xBFFA4020) Some or all of the samples requested have not yet been acquired.
To wait for the samples to become available, use a longer read timeout or read later in your program. If your session uses a trigger, make sure that your trigger is configured correctly.
Status Code: -200284
Regards,
Chris Delvizis
National Instruments