We have a batchable Apex job (process) where we call an external web service for some Accounts. It starts and service is called. Then, for some records it we get the error : Unable to tunnel through proxy. Proxy returns "HTTP/1.1 503 Service Unavailable". Then after some time it gets ok. Any ideas what could be the issue or how to troubleshoot?
This error happens if your firewall doesn’t allow access to your server where the web service is hosted. Making the firewall changes should fix this issue.
In general, when you see an issue, follow this checklist to troubleshoot the issue: Make sure you have the firewall settings in your enterprise is configured to allow the inbound web service call. Check whether the certificate is expired or not. Check if your chain of trust has valid certificates.