add audio to video crashes Cubase

Hi, I tried to replace audio in a video file with Cubase but it always crashes Cubase. I have a 24 bit 44.1 mixdown that I want to add to a .mov file (H264 codec). Before starting my session, I made sure that the framerate in project setup was the same as the video. First it just crashed, with the second attempt I got a message something like this: “Error: medium type not supported or invalid medium”

Anyone else who has encountered this and knows perhaps a solution? I used a different program to add the audio to the video, but the sync is not as good as in Cubase itself…