ERR_CONNECTION_CLOSED Error

Hello,

I haven't made any changes from fortigate for a while and it was working fine, but this morning I got a response from users saying they were getting an error saying they didn't have an internet connection when they tried to access youtube. When I checked, there was no error log in fortigate, but when I checked from chrome, I saw an ERR_CONNECTION_CLOSED error in some requests made to googlevideo and youtube domains. After a long effort, I saw that access was fixed when I allowed these domains in the web filter, but now the same thing is happening for yandex etc. What could be the reason? I can't see any deny log in FortiGate.