Hi @LukasFrey, any chance you can test this against an n8n instance running outside of your current environment without a proxy? This is to verify whether it is indeed your specific setup using N8N_USE_DEPRECATED_REQUEST_LIB=true and your proxy that is causing the problem here.
In general, the current library used by n8n for HTTP requests (Axios) should support proxies. @Jon tested this recently over here, so it might be worth a shot removing this option (it will also become unavailable with the next update). Just make sure you have set http_proxy and https_proxy instead.
If this still fails for you, can you confirm which proxy exactly you are using and how its configured, so I can verify whether this breaks OAuth2 for me as well?