Kuzma Lesnoy

No specific "why" and solution was not in it in the end, just tried everything, but it could cause interconnectivity issues.

Whoa, solved for me. Yes, I forbid IPV6 traffic and blocked port 7680 for outbound connections. This port is used by Windows Update Delivery Optimization (P2P) so I turned it off too. Apparently, my ISP saw aggressive P2P connections on this port and flagged me for "spamming" traffic, firewall actively choked bandwidth and began dropping network connections entirely to protect itself. Interesting, that it caused troubles always and only in SimRail.

Any updates? Have the same issue :(