system
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Hello!
Thank you for the reply.
I was using Eset, but uninstalled it before installing Avast. No other firewall as well. I restarted the modem but nothing changed.
However, when I checked the Device Manager, it showed multiple devices which I didn’t recognize. I deleted them, rebooted and now everything works fine, including Avast 
I don’t know what the connection might be, but I hope this info helps anyone who might have a similar problem.