Exactly as described in this thread.
disabling ECN by “echo 0 > /proc/sys/net/ipv4/tcp_ecn” solves the problem, but only until the next restart as this change is only temporary.
in short all websites protected by Cloudflare return ERR_CONNECTION_RESET. The same on all different browsers.
The ISP provider is Comnet BG
The answers in the posted thread suggest an issue on your end.
matrix.org is one of the affected websites.
Let me know what other debug info you need and will be happy to provide.