When you get the errors about the MySQL-Link it is because of the connection file. If the values in your connection file have different names, or you are missing a part of it you can get this error message. You can also get this error message if the code that makes use of the connection file occurs before the connection file is included.
Are these the errors you are getting on the checkout failure page? If so you should take a look at the page and make sure that you have the incude for the connection file above the lines of code that are giving you this error.
