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FortiGate Issue - Virtual Server - 504 Gateway Timeout with firmware 7.6.5 and 7.6.6

Forum|Forum|6 months ago
February 1, 2026
19 replies
2837 views

Hello everyone,

Recently I’ve encountered issues with FortiOS 7.6.5 and 7.6.6.

We are running an on‑premises Exchange Server with access via OWA/ECP.
Instead of a regular VIP, we publish this using a Virtual Server (load balancer) on the FortiGate.

No ZTNA, FortiProxy, Endpoint Management, just a FortiGate with access to Exchange.

The setup is straightforward: HTTPS on port 443, a public record, a wildcard certificate, and a mapping to the real server.
With FortiOS 7.6.4 everything works correctly and OWA/ECP loads as expected.

However, after upgrading to 7.6.5 or 7.6.6, opening the OWA page immediately results in:

504 Gateway Timeout: remote server did not respond to the proxy.

No changes have been made to the Exchange Server configuration — only the FortiGate firmware upgrade.

Rolling back to 7.6.4 resolves the issue, but we would prefer to stay on the latest version.

Has anyone experienced this, or does someone have an idea what might cause it?

Best regards,
Tim

FortiGate

funkylicious

SuperUser

hi,

maybe due to https://docs.fortinet.com/document/fortigate/7.6.5/fortios-release-notes/78623

"jack of all trades, master of none"

tim86Author

New Member

Hi!

Thank you, I will test this as soon as possible!

live89

Explorer III

have you found the solution?

i am encountering the same

nunoruivo

New Member

I'm having the same problem. After updating FortiOS from version v7.4.11 to version v7.6.6, I can't access web pages published on two servers. The most interesting thing is that it only fails when the server uses IIS; another server with Apache works perfectly.
Have you managed to resolve the situation?

live89

Explorer III

I think we all should open tickets to TAC, Then this will escalate quickly.

I opened one yesterday regarding same issue.

If more open tickets they will take it seriously. They don't spend much time in this community.

HarryTran

Staff

Hi @live89
Could you share me the ticket number, I will have a look on it.

Thanks,

Harry

renegadealien

Visitor III

I am experiencing the same issue.

There are 10 different websites hosted on one virtual server.

4 of them are on one server, and 5 of them are on the other server.

The websites on the first server are working, but the ones on the second server are showing the error: "504 Gateway Timeout: remote server did not respond to the proxy."

It is not related to the change described at
https://docs.fortinet.com/document/fortigate/7.6.5/fortios-release-notes/78623
— I tried that.

Is there anyone who has been able to fix the issue?

live89

Explorer III

I also tried it and did not work. I am still waiting for Fortinet TAC to "reproduce the issue" next week.

I also seen many talking about issues similar to this none on reddit:

https://www.reddit.com/r/fortinet/comments/1r142w9/fortios_766_upgrade_notes_and_review/

Still waiting for Fortinet to come with a solution, or at least tell us the root cause.

tim86Author

New Member

Fortinet has confirmed that they have been able to reproduce this issue. They requested to perform additional tests in a production environment, which was not feasible on my side. It is clear that this is a firmware-related problem that needs to be addressed by Fortinet. The issue first appeared in version 7.6.5 and is still present in 7.6.6. Given that more users are now experiencing the same problem, I would expect a more proactive response from Fortinet.