DevOps, CI/CD and Automation

Not sure whether this pertains to VS 2010 or to the upgraded framework, but... we are using the Oracle membership provider to authenticate users. Prior to the upgrade everything worked fine, but now Membership.ValidateUser(user, password) returns false despite valid credentials. There is no exception thrown, so it's hard to determine what the problem might be. The website administration tool in VS 2010 is still able to manage users and roles, so I have no reason to question connectivity. What might the problem be? There is so little code apart from the Web.config <membership> block, I don't know how to troubleshoot.