how could it solve.

The reason I asked if you could send and receive emails was to determine if you had a working connection to the internet, and you do. I suggest that you next restart the computer in the Safe Mode. Hold down the Shift key on restart. Be patient as it can take some time while the operating systems repairing itself and cleaning its caches. Then see what happens. Then restart normally and retest.

your tagline says you are using macOS 10.12 Sierra. is that correct? if yes, that may be the issue. safari on Sierra is many versions behind the current safari, and can cause the issues you are facing. you can try a different browser. something like the ESR version of Firefox could possibly do the trick.

What happens when you try? Do you have the ability to send and receive emails? When did the problem occur? Was it after anything was changed? We need more information to evaluate for a response.