FortiOS 6.4.3 problems!

Hello everyone, I am managing a Fortigate 60E, which up until yesterday, was running  FortiOS 6.2.1 build 0932(GA) for many months without any issue. I backed up the configuration and upgraded the system with the latest firmware and patches, following the proper upgrade path up to FortiOS v6.4.3 build1778 (GA) so the system is up to date. Ever since the upgrade, users complain about websites being blocked, general internet browsing delay and out of nowhere, "encrypted network traffic, untrusted certificate" prompt messages from their ESET antivirus, which i am not sure how relevant to our Fortigate device is but users claim that they started having these problems today which is after the FortiOS upgrade. Most of the websites being blocked have already been set up in the Policy & Objects ----> Addresses in the FQDN section to have access before the upgrade

I am running out of ideas atm and i am not sure if there is a way to properly downgrade the system back to 6.2.2 without creating bigger problems

Currently i am forced to turn Web filter off so everyone can work but this is not a solution of course.

Any ideas or anyone with similar problem ? Regards