YouTube playback error

Go to Safari Preferences/Extensions and turn all extensions off. Quit/reopen Safari and test. If okay, turn the extensions on one by one until you figure out what extension is causing the problem.

Uninstalling AdBlock app.

https://help.getadblock.com/support/solutions/articles/6000198750#uninstall

I think Safari has some problems because I browse YouTube with my AdBlocker turned on in FireFox and it works fine. Also, I have a different Windows machine which has Chrome on it with AdBlock Plus and it works fine as well. You should try by disabling AdBlock first, then disabling the extensions one by one. If it doesn't help then try to reset the factory settings in the browser. Let me know if that works.

Thank you

I would delete the Kaspersky extension. There are no Mac viruses at this time and ant-virus software causes more problems than it helps.