This looks like a confirmed n8n Cloud bug rather than an issue with your OpenAI API key or setup. Multiple users are reporting the same config.headers.setContentType is not a function error after recent cloud updates, especially with OpenAI credentials and HTTP Request nodes. (n8n Community)
For now, your best option is probably:
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Open a support ticket with n8n
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Include your n8n version, workspace URL, and the exact error
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Reference the existing GitHub issue so support can link it faster
Some users mentioned using the HTTP Request node as a temporary workaround until the fix is released. (n8n Community)
Funny enough, bugs like this always seem to appear right when someone is automating something important like a university admission workflow