No, no solutions yet.
As one of my devices I'm using is relatedly new and still under AppleCare, I phoned Apple Support about this.
We went through the usual basic stuff… they took over my screen and did some screensharing, so they could see exactly what was going on. I quickly got bumped up from Level 1 Support, to Level 2 Support and eventually to Level 3 Support, which is where I'm at for the time being.
So far, all of the following have failed:
- Reboot (natch)
- Try it in Safe Boot (which fully re-enabled SIP, that I'd partially disabled… so it's not that)
- Try it logged in to a separate, temporary user account
- Reinstall iTunes
- …and just now, Reinstall OS X from the Recovery partition!
None have worked.
I have to say, I had high hopes for the last of these, as I was beginning to think it was something I'd done myself. I have in the past removed iBooks from the OS on both my machines (I hate it, and would much rather manage all the metadata of my (many) Audiobooks & Books inside of iTunes – there's little or no scope for doing this inside iBooks). When I looked in the Console.app, I saw references to "BKAgent", which is one of the files that I think gets removed in the process of uninstalling iBooks.
However, after I've reinstalled El Capitan, iBooks is there, yet the iTunes Store is still completely un-functional on both of my devices. The only things I've got in the console now, that could mean anything are:
com.apple.xpc.launchd[1]: (com.apple.xpc.launchd.domain.pid.MediaLibraryService.535) Path not allowed in target domain: type = pid, path = /Library/Frameworks/iTunesLibrary.framework/Versions/A/XPCServices/com.apple.iT unesLibraryService.xpc error = 147: The specified service did not ship in the requestor's bundle, origin = /System/Library/Frameworks/MediaLibrary.framework/Versions/A/XPCServices/com.ap ple.MediaLibraryService.xpc
iTunes[468]: __tcp_connection_get_statistics_block_invoke 59 connection is not established
Back to my Level 3 Support guy, I guess!
I'll post more when I know more.