When browsing through the Web Gateway in Proxy mode with DLP reporting enabled, you see the following error message.
Packet capture evidence shows that the DLP Network Prevent for Web server sends error code ICAP/1.0 503 Service overloaded.
The DLP server is hitting the configured limit on the number of ICAP connections that it can accept.
You can safely click Back and the web page should be available. This behavior will be changed in a future release of the Web Gateway.
To prevent future occurrences, the setting for how many ICAP connections can be made needs to be increased or more DLP Network Prevent for Web servers need to be deployed to handle the load.
Please review the DLP Network Prevent for Web ICAP Performance Tuning kb document here: https://support.symantec.com/en_US/article.TECH219293.html