please excuse the late reply, somehow I had overlooked your last mail, and I have not been doing video editing work in the last few months.
I use the MacOS version of Video Splitter, I currently have version 3.3 installed, and I still get the error message that the source file is not supported or damaged. QuickTime Player 10.4 is playing and editing the file without problems, but as most of my files are around 70 GB large it takes ages to edit them, as QTP is always making a copy of the section between the in- and out-markers. I want to use Video Splitter to trim the first and last few minutes of many 4k-videos, which are each about 60 to 120 minutes long.