Mac mini cannot login A newly bought Mac mini was working perfectly for 4 days with a single user. Now when I tried to login, the screen turns blank (the Apple icon appears when first started, with that background color) for a moment and jump back to the login screen again.
When I try the wrong password entry, then I get the known shake-effect as expected. 

What is the reason I cannot login and how can i solve this?
Try using Recovery Mode to correct this issue. Change the affected user password with Recovery Mode and see if it works. If this doesn't work, you need to return this Mac to the Apple Store and have a Mac Genius check it out. It will save you many headaches to let them check it than trying to solve the issue yourself.