QuickTime Stops "New Movie Recording" When Recording From an External Source

I'm trying to digitize some old MiniDV tapes using QuickTime's "New Movie Recording" feature. I have a Sony deck connected via thunderbolt. QuickTime recognizes the source deck's video and audio signal and even starts to record a few seconds of the video, but inevitably within a few seconds it stops recording and throws the message: "Recording stopped. Try recording again." This happens no matter what the quality settings is (high or maximum). It happens with the "Enable Power Nap" settings off in Energy Preferences. It happens in Safe mode. It happens on an older Mac running Sierra 10.12.16 and a newer iMac running Sonoma 14.3.1. It does NOT happen when the source signal is the iMac's built in camera and microphone.

Any suggestions would be greatly appreciated. This shouldn't be so difficult!!