I'm using a Windows PC. All three installations at my house are on Windows PCs, and I have a Windows license (not a Master License). I used to have a Mac, but switched to PC earlier this year.
I just changed, for my Silver computer, Red October to Red October JRVR. (Just checked, Black computer is Red October standard) Update soon.
I haven't compared the settings between the computers yet, but I will. I think they should all be the same since we did not perform a Custom installation on any of the three computers.
All media files are stored on my NAS. On my Silver computer (the computer giving me these problems) NAS is mapped as S:\. On my Black computer NAS is also mapped as S:\. I think the same is true of my son's computer, but I'll have to double check when he gets home.
One difference is my Silver computer is hard-wired to our NAS, the other two computers connect via our wireless router (same router, same NAS, just a wi-fi connection rather than ethernet).
After you asked the question about MC28 being able to read my video files I remembered I'd reinstalled MC a couple of times and didn't change any of the default file formats. I went to Tools -> Options -> File Formats and checked the File Associations box for .mkv. It didn't help, though, I still have to exit MC28, wait for five minutes or so and then play the next video I want to see.