You may be experiencing machineKey disparity.
In web.config - the implicit
machineKey section uses autogenerated encryption and validation keys that are again keyed against the app Id (
Are you testing with a different application instance or on another machine?
Is the Oracle membership provider 2.0 or 4.0?
And this is just a stab in the dark, maybe this will lead you to a solution.
From .NET Framework 4 Migration Issues
Some types (for example, System.Web.Security.MembershipProvider) that are used in ASP.NET membership have been moved from System.Web.dll to the System.Web.ApplicationServices.dll assembly. The types were moved in order to resolve architectural layering dependencies between types in the client and in extended .NET Framework SKUs.
Class libraries that have been upgraded from earlier versions of ASP.NET and that use membership types that have been moved might fail to compile when used in an ASP.NET 4 project. If so, add a reference in the class library project to System.Web.ApplicationServices.dll