Sort of a non-post this one, but I'll put it here in case anyone else has the same problem.
I just updated to 18.175 and i noted that madVR was no longer being used as the video renderer. Every time I opened a video, the 'adding required components' would flash up on the screen (as if trying to download each time but failing), but ROHQ would always fall back to EVR as the video render(quite gracefully I might add).
I tried to manually install the new madVR in the appropriate mc folder, but found the folder 'still in use' by something even though MC and MC server were closed.
Answer: I found the folder was still in use by the madVR tray icon. For some reason I'd enabled this in the madVR settings and it was blocking the update. Exiting the tray icon fixed the problem and MC then auto-updated the madVR files.
Moral: Don't change the default settings (unless you know what you're doing!) [actually I can't think of a good reason to have the tray icon activated]
SBR
PS Maybe one for Jim's weird and wonderful thread.