I am having a hard time keeping up.
First there was a LCD problem with IDE 1.5.4
Then the LCD works on another arduino board.
Then your arduino had only 3.3 volts on the 5 volt pin.
Now you get Compile errors when you use a RTC shield.
Lets slow down. The last thing I mentioned was that you had a problem with your power supply ( 5v). Have you got that fixed yet.
You replied
I use a 5 V DC power supply connected to VIN now and got stable conditions on the shield.
. I don't really know what that means. Do you read 5.1 Volts on the 5volt pin now?