I have an iMac with Mac OS X 10.8 (Mountain Lion). When I open the Mac App store, and try to download an App, it asks me for the login. When I enter my Apple ID's email and password and click login, the button become gray for a moment, and then asks me for the credentials again. There is no error message displayed. The email and passwords are correct, since if I try an incorrect password it tells me that it is wrong.
MacOS – Can’t login to Mac App store
apple-idicloudmac-appstoremacos
Related Solutions
It looks like that User's folder got corrupted somehow, and the System doesn't know what to show and load, as It wasn't there.
Try logging with other user or a Disk Repair / Repair Permissions, through the Recovery Partition.
EDIT: Since you made your edit my answer is not valid.
In Apple store click on Store + View my Account
Sign in.
Bottom right just below the DONE click on the globe flag icon, and change to your country.
Best Answer
I've solved installing the manual updates for Mountain Lion from here, and then it began to work normally!