[Milhouse]

That did it. Set threads to 1 and it works. Any idea why this is? Is it a docker restriction you think or computer hardware running docker?

Nope, no idea, is the Emby server process multithreaded or able to handle more than a single request at a time? Seems like it might not be. You'll need to throw this over to the Emby developers, it's their software that's the problem.

Also, is there ever a time where I would need to run big c over little c? Like a use case, since big c is disabled unless overridden, just curious.

"C" shouldn't be needed unless you're experiencing a problem with artwork for specific movies, tvshows or music etc. and you need to re-cache your artwork (for example if it's recently changed locally, or the cached version is corrupt). I certainly don't recommend using it en masse.