I have a same issue with Shotcut version 24.02.29
Fortunately, I kept the previous version of Shotcut version 22.
It looks like the hardware decoder is not being detected and is probably related to multiple video cards.
last time, a similar problem was solved by enabling the video output duplication mode.
But this time, when the new version tries to find the decoder, it says “nothing found”, although version 22 defines the h264* decoder