QuickTime permission error saving video after two folders

Hello lovely people, I have a dilemma that I haven't had before. I've done a search and have seen a number of answers but none that appear to be quite the same as my issue.

I have created multiple image files (doing a Timelapse) and have used Quicktime to create a short video from 999 image files). I can do 2 folders (out of a total of 10) but upon trying to save the file from the third folder, I get the warning notice that I don't have permission to save. I have read and write permissions for everything on my iMac, so there is clearly some glitch somewhere that triggers this. When I first started this I initially got around it by restarting the iMac, but this then fails on the third folder and I would have to restart the machine again, a laborious and idiotic way to maintain a workflow. I got around the problem after that by making a copy of the video file but now, Quicktime won't even create a video from the files. Having selected all the files in the folder, in Quicktime, it instantly creates a new video file made up of just one image, not all the images in the folder!!! I'v tried 3 times and it does the same thing each time.

Can anyone offer a clue as to what may cause this and a solution, please?