rfielder: you probably tried to connect first day when there was server and no upgrade message. Now it should be ok. So answer is, server is 3.1.1 compatible.
Abou second question 3.1.1 and 3.1.0 cooperation. Answer is now, but next i will consider some solution for such situation. Because i must increase version just because of private games on online server. Technically for direct games there is no change (but protocol version is now same for both)
Anywat I recommend to upgrade, changes:
online play - password protected games support
tile color adjustment (Jozsef T)
updated es translation (outdated in 3.1.0)
error message bar can be dismissed
rotate board (/ key)
right click on board with shift down behave as middle mouse click (center board to click point)
fix: too often AI placement on top of tower
fix: "2-tiles bug" - if playing with GQ11 & King and Scout - at game and counter still shows 2 tiles
fix: can't place on tower if there isn't normal placement possibility
fix: It is not possible to place the phantom as an abbot on a German monastery
fix: checking show farm hints option before game starts is broken
so it's almost all bug reports and easy improvemens from this discussion. (and contains middle button simulation for you