Because of a change to the map file format (that means buildings can now be placed anywhere in the world, not just on the corners of tiles), the new servers are not backward compatable - i.e. old clients will not be able to connect new servers, and vice versa.
All players should be updated to 0.51.0 over the next day or two, so update your servers now otherwise your players wont be able to connect.
cheers
