The black screen in GSpot shows the codecs aren't working. If they don't work in GSport, they won't work in TandemVids either. It's a complicated issue and I don't really have a solution. I've spent hours and hours trying to fix broken codecs on customer machines without success. If the LAV Filters don't repair the broken codecs, there is something amiss. It probably stems from some other software that hasn't uninstalled correctly. A dodgy or unregistered codec is taking over and failing to render the footage. You have to work out which codec it is and disable or remove it.
I know TandemVids works fine with the LAV Filters on all supported newly installed operating systems. TandemVids does not modify or change any codecs.
I don't know what to suggest but, if it was my computer, I would probably wipe it and reinstall the operating system. You have to way up the hours wasted messing around with codecs versus the few hours it takes to rebuild a computer.
If you persist, there are plenty of forums and help sites dedicated to codec issues.
Mick