Posting more complete error messages can increase the chance. Fortunately I found a possible example:
Jun 25, 2018 9:30 AM: Failed to send message: System.Net.Sockets.SocketException: A socket operation was attempted to an unreachable network 184.108.40.206:53
"Message":"A socket operation was attempted to an unreachable network",
If that’s like what you saw, then WSAENETUNREACH (10051) Network is unreachable. may explain it as something in the network reporting it couldn’t get to the server. Possibly your different connections take a different path to the server. You could run the tracert or traceroute command to see what route it’s taking.
Though it’s a rather advanced technique, Wireshark could give an even better view of packets and errors.
The “different internet connection” isn’t real clear, but I’m assuming it refers to a different site and network. Are there commonalities among the working network connections, and differences from those that failed?