This can have several causes. First of all you should check if you have entered the username and password entered in PLESK correctly in your script / application. If the database access should be local on the server, please use localhost as database server. For external access, the correct server name in the form hostingxy.nbgo.de must be used and the database must be enabled for external access.
If this does not solve the problem, please contact our support. They will be happy to help you with your problem.