Sometimes, the issue is caused by filedescriptors definition This article is for Artica v4.30x SP206/208 LTS, if you use an higuer version, use this article
Filedescriptors settings can be modified using:
On the left menu, choose “Your Proxy” / “Global settings”
Click on “Performance” tab.
Check if your Filedescriptors status are not in red
On this Artica server, the server run out of filedescriptors, Active Directory connections and surf are unstable.
If it is the case, modify values in the Filedescriptors ( proxy ) and Filedescriptors (system ) field. Filedescriptors (system ) must always be higuer or same value than Filedescriptors ( proxy )
Click on Apply, the proxy service will be restarted.