Mac App Store Login Problem

Since I upgraded my MacBook to Mountain Lion I have the following problem: in the Mac App Store it's not possible to log in or to buy/download anything. Clicking on log in just doesn't do anything, no error massage. What to do?

Edit: The following error messages are in the log file after trying to download some app:

08/11/12 20:36:57.171 -[ISStoreURLOperation _runURLOperation] - load the URL from the bag if needed
08/11/12 20:36:57.172 url:http://ax.init.itunes.apple.com/bag.xml?ix=5  bagKey:(null)
08/11/12 20:36:57.173 -[ISStoreURLOperation _runURLOperation]: _addStandardQueryParametersForURL: http://ax.init.itunes.apple.com/bag.xml?ix=5
08/11/12 20:36:57.179 ISURLBag: Insufficient data to parse URL bag
08/11/12 20:36:57.181 ISStoreURLOperation: Chose not to retry after error: Error Domain=NSURLErrorDomain Code=-1003 "Unknown Error." UserInfo=0x7f8cbb060600 {NSLocalizedDescription=Unknown Error.}
08/11/12 20:36:57.182 -[ISStoreURLOperation _runURLOperation] - load the URL from the bag if needed
08/11/12 20:36:57.182 url:http://phobos.apple.com/bag.xml?ix=5  bagKey:(null)
08/11/12 20:36:57.182 -[ISStoreURLOperation _runURLOperation]: _addStandardQueryParametersForURL: http://phobos.apple.com/bag.xml?ix=5
08/11/12 20:36:57.188 ISURLBag: Insufficient data to parse URL bag
08/11/12 20:36:57.189 ISStoreURLOperation: Chose not to retry after error: Error Domain=NSURLErrorDomain Code=-1003 "Unknown Error." UserInfo=0x7f8cbb062270 {NSLocalizedDescription=Unknown Error.}
08/11/12 20:36:57.189 ISAuthenticationChallengeOperation: Could not load URL bag: Error Domain=NSURLErrorDomain Code=-1003 "Unknown Error." UserInfo=0x7f8cbb062270 {NSLocalizedDescription=Unknown Error.}
08/11/12 20:36:57.190 AuthenticateOperation: Authentication challenge failed: Error Domain=ISErrorDomain Code=-128 "Unknown Error." UserInfo=0x7f8cbb031150 {NSLocalizedDescription=Unknown Error.}
08/11/12 20:36:57.192 StorePurchaseBatchOperation: Canceling 1 purchases after failure: Error Domain=ISErrorDomain Code=-128 "Unknown Error." UserInfo=0x7f8cbb031150 {NSLocalizedDescription=Unknown Error.}