I tried to exclude some URL's (the webpage URL, the WS server's URL with ws:// prefix and the same one with http prefix) and the problem persists.
The report file is empty, there are only start/stop logs.
Repair done, same problem.
@Nesivos thx for the links but I'm already aware about this situation. I enabled WS in firefox and chrome by disabling the default security setting etc, the problem is not there. Moreover when I disable avast web shield it works, so it's not a browser's configuration problem.