Like many of you here I ran into this problem just this past weekend.
After researching the problem myself, I believe that I may have inadvertently caused it.
This is how.
My iMac (which is up to date with OSX Mavericks and the latest version of iTunes) was only being used by me. However this weekend I backed up my GF's old iPad Mini to my computer. When she came home with her new iPad Air, I just restored it from the backup I had taken just a few hours before.
The turning point, I believe was when backing up her iPad Mini, it transfered all her app purchases onto my iMac, thus somehow corrupting some setting.
I then purchased an album, not from iTunes btw and imported it to my iTunes library, like I've done so many times in the past. I then waited for iTunes match to sync it, so that it would be available on my other iDevices.
Nothing happened!
I then went to iTunes and manually asked it to sync to iCloud. That's when that now infamous message came up about the computer being only associated with one Apple ID and that 90 day limit when trying to associate my iMac with iTunes Match.
I am now exchanging e-mails with an Apple support person and they hopefully may find a solution to my problem.
I will keep you posted if my problem is resolved.
P.S. The rest of my music in the cloud works problem free on my other devices, including playlists etc. (iPhone, iPad, Apple TV). At least that isn't broken.