Unable to re-add FortiGate to FortiManager Cloud - Certificate Serial Number Mismatch Error

Forum|Forum|7 months ago
January 16, 2026
3 replies
629 views

I am running into an issue re-adding a FortiGate (running v7.4.8) to my FortiManager Cloud. The device was previously managed by this FMG but was removed temporarily for maintenance.

Now, when I try to re-link them from the FortiGate side (Security Fabric > Fabric Connectors > FortiManager), I receive the following error:

"The FortiManager's access to the FortiGate will be authenticated by the FortiManager certificate. The serial number from the certificate must match the serial number observed on the FortiManager. Could not connect to the FortiManager to retrieve its serial number."

joshbergm

Explorer

Hi!

Is the entry still visible in the FortiManager Cloud? You can try to delete the FortiGate in FortiManager Cloud and re-add it after.

ZohebSkAuthor

New Member

Entry is not visible in fortimanager cloud

farhanahmed

Staff

@ZohebSk Usually this error indicates the FGT is not able to reach FMG.

1. Try setting fmg-source-ip on the FGT and see if that helps.

2. Make sure the FGT can resolve 'fortimanager.forticloud.com'.
3. Find the IP of your FMG Cloud instance and run sniffer to see if there are any issues.

4. Also run the FGFM debugs on the FGT and see if it shows any details.

Refer to the KB article to tshoot:
https://community.fortinet.com/t5/FortiManager/Troubleshooting-Tip-How-to-connect-FortiGate-to-FortiManager/ta-p/328728
https://community.fortinet.com/t5/FortiManager/Troubleshooting-Tip-How-to-troubleshoot-connectivity-to/ta-p/337527

farhan.ch