I have the same problem - at some point my Mac decided that it's already associated with another Apple ID. I have one Apple ID, it's the one I subscribed to Match with.
I have multiple Mac accounts (for the kids) and they have their own Apple IDs, but not iTunes Match accounts and they can't use my Apple ID, so it's impossible that they've activated iTunes Match (and I've checked that they haven't as well.)
The impression I get is that iTunes Match just won't work on a machine with multiple users who all use iTunes with different Apple IDs, so I'm abandoning it as a bad idea unless I can find that I'm wrong. Nothing on this forum suggests anythign else so far.
Unfortunately I can't find anyone who actually understands how it works, or how I can find out WHAT Apple ID iTunes thinks is associated with Match - this information isn't available anywhere in my account.
I have got to the bottom of this...
In short, any one device can only be associated with one AppleID if any of the following are used:
- iTunes Match
- Automatic Downloads
- Prior Purchase Downloads
In my case my daughter activated Automatic Downloads on the Mac and suddenly the Mac is linked to her account for a minimum of 90 days!
This is obviously not good, and is totally impractical in real life.
I will winge at Apple and see what happens.
+1 with a similar problem here.
Visiting family for XMas, and have an account on their multi-user iMac (Apple, you made a big thing of the Unix heritage, didn't you? Well guess what - that means multiple users on each machine).
Was given a CD for XMas. Have an iPhone5, and had previously signed up to iTunes Match on my home machine (Mac Mini). Sudden brainwave: import CD into my account on the iMac, then use iTunes Match (already paid for) to get songs onto iPhone without any faffing about.
Ran straight into "This computer is already associated with an Apple ID" nonsense. It is simply outrageous that Apple are locking iTunes Match to a machine rather than an account ON each machine - they really don't seem to grok "cloud" at all, do they?
Needless to say, I am pretty disgusted. As a poster above notes, Google are starting to look more attractive by the day - Apple are clearly headed for disaster with the path they've chosen.
Direct impact: I am switching off the auto-renewal on iTunes Match, because there is no way I'm paying another year for this level of "Service".