AFAIK this means that the connection between you and the "login server" is bad or at least not stable. As usual, that could be caused on the server end (then everybody is affected), on your end (only you are affected) or in between (some/many people are affected).
I've had this problem very rarely (when it wasn't caused by the server), but usually it was gone after a few hours, and I had no real idea why it happened. One possible thing to do is to try another browser (unlikely), another device or, if you can, another route to the internet (say using an access point that has a DSL/cable/other "line" connection to the internet, vs. a wireless one base on mobile cells.
Did you change any network configuration lately, or added a VPN or security solution (like an Anti-Virus program)?