I got this too when trying to generate a Python SDK from an OAS 2.0 version JSON file, using the Generate Client at https://editor.swagger.io/
It seems the tool is broken for any OAS 2.0 version and a 520Byte .zip file is generated containing an HTP 500 error, like the one you got.
Support told me it's broken temporarily and that they are introducing 'enhanced functionality exclusive to SmartBear API Hub plans', so it may be something to do with that and the move to the new tool which supports OAS 3.0 and greater.
It's a pity, as this was a great free tool which sounds like it may sadly no longer be so in future.