(Noam -- I hope it's ok if we redirect the email thread here)

It seems that the most likely reason for this error (that I can think of) could be due to the display device.

If this is the case --
Either the display device is having trouble properly rendering both videos together in DirectRT's native frame-precise environment,
Or the display drivers being used with the display device are outdated.

I would first recommend seeing if there are possibly more recent drivers for your machine that could be downloaded from the manufacturer.
If your display drivers are up to date, we may want to try connecting through a support session to see if I can find an alternative solution to the problem.