iTunes, An unknown error occurred (502)

You could be right, and I can't discount the, ahem, "challenging" nature of iTunes today, but there is much network voodoo going on in the cloud of which even network admins are unaware. For example, accessing a game download, a sign-in to a PSN ID, and various other services from the Playstation Network can work, while only certain demos, and movie rentals will not (personal experience.) There are ports, cached files, reroutes, and mumbo-jumbo at work here that can rarely be explained by 1st and 2nd-tier technical support, and would require deep data center investigation to obtain more information.

9 out of 10 tracks could be hosted at one server in Luxembourg, and the remaining track could be hosted in Alabama. More likely, that one track had some corrupted cache file at your ISP. ISPs do stuff like that so that everyone downloading Bieber's latest album don't have to ALL download it. It just comes from the ISPs cache, or Akamai, or Amazon S2 servers, saving them, and Apple, bandwidth. Be assured that Apple, along with everyone else in the world using cloud services, IS changing settings and configurations on their end which affects many things. This is the nature of the world of software as a service. These things get created, fixed, and altered daily by the network gurus who work there and magically our stuff begins to work again...or not.

Hi guys,

First post but confirming Matt's guess. Running 10.8.5 OSX and 2.2.5 Server. Once Caching service switched of it all of my updates that had 502 error downloaded fine. Also as some suggested using different network worked fine as well. Mystery solved at least for me.

I appreciate your corroboration, but turning off the caching service doesn't solve the problem/mystery. You've worked around the problem by disabling a useful service that—for an unknown reason to us, but probably not to Apple—just punishes you, because now you won't benefit from the caching service. Perhaps you don't need it, and can happily re-download OS releases, patches, and apps for all your devices, but this service saves me *hours* of time weekly.

I agree with fact that Caching Service is quite important and in larger network environment essential. Mystery is still there and in my smaller controlled environment I will most likely switch it off every fortnight for as long as iTunes update get in and then switch it back on. On that note I wander if anyone has had same error on El Capitan and OSX Server 5.0.15. Thinking of dedicating this weekend to make a move to "latest and greatest".

Upgrading is, of course, exactly what Apple wants. However, their best intentions do not change the fact that sometimes computers need to remain static and we, as admins, need MUCH more transparency about how and why a service has suddenly decided to stop working so we have the knowledge we need to decide to use a stable, but unsupported system, or move ahead with buggier, but supported systems.

It is interesting to note that while enabled, the caching service's data size has incremented for Mac App Store downloads, but not for iTunes.

I had this error on my Iphone trying to download Google Maps. Turned off the Wi-fi and used the Cellular data instead and the problem went away. I would suspect the average home router has some ports blocked that Itunes tries to use on some downloads. Though I downloaded 2 other apps through the router with no problem.

Suggest trying different networks or go into your router setup and shut off all firewalls and enable all ports. That might fix your problem. And if Apple reads this i might suggest they look at these problem apps and see what they are doing differently from the rest and try a permanent fix at their end.