We keep getting 503 error when accessing on-prem app via Azure App Proxy

Question

We keep getting 503 error when accessing on-prem app via Azure App Proxy

Admin at PeriGenSolutions 0

Initially things seem to work but in no particular order we end up getting a 503 HTTP Error 503 Service Unavailable. The logs show no particular information as for the root cause.

Admin at PeriGenSolutions 0 Reputation points

2026-03-24T17:14:03.8466667+00:00

We have Developer support. How can we reach an engineer who can help us with this diagnostic?
Rukmini 36,695 Reputation points Microsoft External Staff Moderator

2026-03-24T17:39:50.8866667+00:00
Hello Admin at PeriGenSolutions

The issue 503 occurs when Application Proxy Connector cannot establish or maintain connection to backend application (most common: backend not consistently reachable or connector instability).

To fix the issue, try the below:

Restart Application Proxy Connector service

Verify that the connector server can always access the backend application (no sporadic drops).

Check Event Viewer → AadApplicationProxy → Connector for timeout errors

Verify the stability of port 443 outbound (no proxy/SSL inspection).

Install a minimum of two connectors in the same group (strongly advised).

This is a connector-to-backend connectivity issue, causing intermittent 503—not a service-side issue.

Let me know if any further queries - feel free to reach out!
Rukmini 36,695 Reputation points Microsoft External Staff Moderator

2026-03-25T20:00:03.8933333+00:00

Hello Admin at PeriGenSolutions

Following up to see if the above provided information was helpful. If you have any further queries do let us know.