I experienced this not long ago, I had a few back end, could not resolve host errors previous to this,
But this one simply said, Back end error, got nothing.
I’m not sure why it came up, it could have been a latency/connectivity issue, my internet occasionally goes wonky. I have no mods installed. This is a link to my steam profile, which should have a screen shot of the error as the most recent screenshot in case the picture doesn’t load. https://steamcommunity.com/profiles/76561197973032524/. Hope this helps in some way.