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
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.