I recently have the same problem and had a bit time on my hand to take a closer look on it. Just taking guesses here, but maybe my findings will help someone.
So I play this game for years now with a very bad internet connection (~2mbit/sec) and it ran relativly stable. A few months ago, finally better internet was available at my place and I switched to another provider, now I'm traveling with 50mbit/sec through the internet (don't laugh, that was a huge upgrade for me xD). Surprisingly enough, that was the point when mother decided to randomly disconnect me from the server after a few minutes.
Main difference now is that my new provider only connects me with ipv6, old one used ipv4. I think the SB server or the game itself doesn't support dual stack or direct ipv6 connection (just guessing). So my router uses ds-lite to connect with ipv4 only services what causes a bit of latency and - at least that's what the wireshark logs indicate - package loss every now and then. That's when eventually mother disconnects me.
I can still play with the vpn workaround. Funny thing is, I can use a vpn server in my own country, doesn't has to be one in france. I think that might be because the vpn server uses ipv4 too so the connection between game server and vpn server is stable then.
So, these are my thoughts about this. But whatever, it could also be just my new router that is a little bitch Will test another one soon xD Stay save & healthy everyone