Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
This afternoon (roughly 4:00 - 5:20 PM ET), there was a system issue which resulted in all V3 API requests returning a 500 response.
This has since been resolved and should now be working as expected.
If you continue to experience this error, please feel free to contact us via email at email@example.com, and reference case #28716367.