All USERID are GUIDs Types.
-->> db.WEB_USERACCOUNT.Where(UC => UC.USERID == USERID) <- the problem happens here ("Sequence contains no elements")
This problem happens only with ODP.Net Managed Driver (oracle.manageddataaccess.client).
Not much detail here to reproduce the behavior independently so that it can be debugged. What is the SQL generated by LINQ for managed ODP.NET and unmanaged ODP.NET? You should be able to get the SQL by calling the ToString() method.
For more details on it, see ODP.NET 12c R.2 and Entity Framework 5.0: problem with binding parameters of type Guid
following SQL statement:
SELECT "Extent1"."Key" AS "Key" FROM " Table1" "Extent1" WHERE ("Extent1"."Key" = :p__linq__0).
(:p__linq__0) gets set to NULL
Oracle has been able to reproduce this problem. Bug 18336370 was filed to track this issue.