When trying to execute a NX Open application in NX 12 there is an error message saying:
"Error in external library. See system log for details".
And in the log file this error appears:
"System.MissingMethodException: Method not found: '!!0 System.Array.Empty()'"
Solution This is an indication that the user has an old version of the Microsoft .NET Framework installed. The required version for NX 12 is .NET 4.6. Installing that version should solve the issue.
Notes and References NX 12 Release Notes->NX System Information->Platforms->Operating system requirements->Considerations and caveats
Hardware/Software Configuration Platform: INTL64
OS Version: 764SP1