can't upload or download documents or files on my mac

It started after the last update. I click, and nothing happens. It happens on various sites; on Linkedin, it keeps getting stuck, and then a day or two later, I notice that the system is downloading files. By then, it's too late for me. I restarted the computer, but it only helped for a day or two.

I see on this forum it used to happen before a lot.

How can I fix it permanently?