you can try to deactivate QUIC protocol in your browser.
For example in Opera browser type opera://flags/, search for quic and deactivate it.
if you use a security software, for example Avast, with a QUIC-security-component try to disable it.
Thank you for your help. I have disabled QUIC protocol in Opera but the problem perists. I have noticed that Chromium 91.0.4472.164 installed on the same sustem is also affected.