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The most common reason for this error is a mismatch between Oracle.DataAccess.dll version and the unmanaged ODP.NET DLLs it's picking up from the PATH.
Is this happening when opening a connection or first calling an ODP.NET API? If it's the latter, then it's likely a config issue. If that is the case, I would first try to de-install all ODAC instances on the machine and re-install ODAC 11.2 R5.
The reason that I recommend the de-install is because of ODP.NET 9.2. Back then, Oracle used a different versioning mechanism that may give it precedence over ODP.NET 11.2. Removing all ODAC instances besides the one you want to use removes the need to worry about Oracle.DataAccess.dll picking up the wrong version of its unmanaged DLLs.