I’m sorry to say the problem keeps reoccurring for no good reason. I’ll be working away just fine on a score with the same video for some time then suddenly when I decide to reopen my score in Dorico later on to do more work the video decides to stop working again… and no amount of restarting or rebooting the PC resolves it.
I’m getting a bit tired of having to fully uninstall Dorico, clear all the my preferences and any files then reinstall it all again just to get working video in my projects, there must be good reason this keeps happening (maybe a bug in video playback system) but I haven’t figured it out yet.
Trying a different approach after doing a bit of research on Cubase video issues (as I imagine they use the same engine), I’m now busy converting the video to ProRes format using this tool: Audio Spot - Convert Video, Replace Audio in Video, Converter | audio spot
I’ll report back on whether this works or not (without reinstalling Dorico).