In case You uploaded Your video file in UdK cloud, create a share link for the file (e.g. " https://cloud.udk-berlin.de/s/Ac7RKyKm8CWE4PS "), then add the "magic suffix"
/download?.mp4
to obtain the URL (e.g. "https://cloud.udk-berlin.de/s/Ac7RKyKm8CWE4PS/download?.mp4") which You subsequently paste into "share external video" in BBB.