Well I can't find the threads that discussed fast video playback, so some notes to consider.
When you have two major versions of MC on one PC, and you switch between them, MC will ask to install something. It does that because it needs to install (register actually) that major version's components, as they are major version specific. Do you see that prompt if you switch between major versions, and are you allowing MC to install what it needs? You should. You can't run two major versions, MC20 and MC21, at the same time. Make sure MC20 isn't starting with the PC and running in the background. Options>Startup>Windows Startup>Run on Windows startup> Nothing.
I'm assuming that you have rebooted the PC a couple of times after installing MC21.0.86? It is often a good idea, particularly if there are apparent problems.
It is possible that a reinstall of MC21.0.86 over the top of the existing MC21.0.86 would fix the problem, if there was an issue during the first installation. Best to do it after a clean reboot of the PC.
If none of the above works, can you upload a smallish sample of one of your wtv files that don't work, to OneDrive, Google Drive, or similar. We can test if it plays correctly on our systems then.
By the way I tested using Red October HQ rather than Red October Standard. Which Video Mode are you using? Have you tried using the other mode to see if the files play okay?
Otherwise I am at a loss why this sort of stuff happens, particularly between versions. It could be a problem with DirectX on your PC. You could run DXDiag.exe and see if it reports any problems.
