Fix Citrix NetScaler HTTP1.1 Internal Server Error 43550 – Troubleshooting & Solutions

Recently, I encountered an issue with Citrix ADC/Netscaler NS13.1.51.15.nc. When accessing Citrix desktops through ADC, after entering the account password for authentication, the following error appeared:

Http/1.1 Internal Server Error 43531

As shown in the figure below:

There have been no recent changes to Citrix or the network. I checked the StoreFront certificate and the ADC certificate, and neither had expired. I also verified that the URL entered for the gateway in SF matched the ADC AG URL and that the configurations were all correct.

Understanding Citrix NetScaler HTTP/1.1 Internal Server Error 43550

The HTTP/1.1 Internal Server Error 43550 in Citrix NetScaler (also known as Citrix ADC) indicates a backend communication or configuration failure. This usually happens when:

• Backend servers are unreachable or misconfigured

• SSL certificate bindings are incorrect

• Gateway or load balancing settings are mismatched

• The appliance is overloaded

After troubleshooting without success, I sought assistance. The engineer suggested trying to modify the Citrix Gateway Session Profile in the Web Interface Address's Change FQDN to StoreFront's IP address (if Load Balancing is configured, change it to the StoreFront VIP). 

The reason seems to be that the LB address can't be resolved via DNS, need to check if the ADC is pointing to an IP or DNS.

The steps to fix HTTP/1.1 Internal Server Error 43550

1. Log in to the ADC/Netscaler web console

2. Navigate to Citrix Gateway → Policies → Session → Session Profiles

TIP: AC_OS refers to the client access session profile, and AC_WB refers to the session profile accessed through the browser WEB client visited session profile

3. Click to enter the AC_WB profile, then click "Published Applications", change the FQDN in "Web Interface Address" to the StoreFront IP address (or the SF LB VIP address), and keep other configurations unchanged. The configuration is shown in the figure below:

4. Click "OK" to save, and it will take effect immediately.

5. If users also need to access desktops through the client, perform the same operation on the AC_OS session profile using the method described above.

At this point, users can successfully access desktops and applications through Citrix ADC/Netscaler.

Best Practices to Prevent Error 43550

• Always test backend server connectivity after configuration changes.

• Keep NetScaler firmware updated to avoid known bugs.

• Enable logging and monitoring to detect issues early.

• Maintain proper SSL/TLS configurations for security and stability.