I suspect network issues, as the server "unexpectedly dropped the connection". This problem wasn't caused by the server not being found, or being too slow, it was because the connection was broken.
WiFi?
ADSL broadband?
I can recommend PingPlotter for diagnosing network problems:
https://www.pingplotter.com/download/macosIf you run that next time you spot slowness, or have it running before you try logging in each time, it'll tell you what the route from your machine to the server looks like. You need to look for slow links, and/or packet loss starting at a particular link.