[Ethereum] Mist: error no data is deployed on this contract address

When creating a new contract via deploy new contract (in Mist on the Test-Net), I get the error "No data is deployed on this contract address", but I filled in every field. it is a very easy straight forward contract:

pragma solidity 0.4.8;
contract MyContract {
 string public userName;
 function MyContract (string _name) {
   userName = _name;  
 }  
}

It compiles without any problem, I can select the contract and I can add the constructor parameter (_name), I can deploy it and send it, but then I get that data error and the contract doesn't appear in my custom contracts overview. I already tried setting select fee on faster, this didn't help, unfortunately.

Anyone having any an idea what this could be? Thanks in advance or your help!