A "Bad Request - Invalid URL" error means that the OpenURL Resolver link is formed incorrectly. A very common error is to leave off the question mark at the end of the URL prefix (ie. http://<resolver domain name>.com/openurl? vs. http://<resolver domain name>.com/openurl ). Try adding a question mark to the end of your URL and if problems continue, contact OCLC Support.
There are steps for managing your OpenURL resolvers in Service Configuration.