Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Apple Release iTunes 10.5.1 beta 3 Fixing iTunes Match –
Apple release beta 3 of iTunes 10.5.1 to developers which is reported to have dramatically improved stability . The beta is available to all developers via the developers centre but iTunes Match beta is only available to developers in the United States. iTunes Match was expected to be released end of October but that release date was missed , with betas coming out more often we believe this is a sign we will be getting iTunes Match soon.
Email to developers :
iTunes 10.5.1 beta 3 is now available. This update includes a number of important stability and performance improvements for iTunes Match. iTunes Match stores your music library in iCloud and allows you to enjoy your collection from anywhere, any time, on your iPhone, iPad, iPod touch, computer or Apple TV. In order to subscribe to iTunes Match, or continue using your existing subscription, please upgrade to iTunes 10.5.1 beta 3. Once upgraded, turn on iTunes Match on each of your computers and iOS devices. On your computer, choose Store > Turn on iTunes Match. On your iOS device, tap Settings > Music, then turn on iTunes Match. On your Apple TV, please turn off iTunes Match under Settings > General > iTunes Store. The songs on your computer should not be affected. As always, please backup regularly and do not delete the music you add to iCloud from your computer.