error while signing: Key not valid ?

[Chris11]

Does anyone know what this error means? And how to fix it?

"Error 2 An error occurred while signing: Key not valid for use in specified state."

I found this solution on the internet but I have no idea how to do it...

"Delete all data sources in the Server Explorer and add them again. Run the wizard again."

 

[Chris11]

Anyone? This is really bugging me...