Can no longer connect to game on local network after update

With two PCs on a local network, it was possible for one player to join the other's world, up until the update to 1.50 auto installed on both PCs.  Now when one PC hosts a world and the other one joins, minecraft exits on the host PC without any error message.  When attempting the reverse , the client PC gets message 'unable to connect to world'.  No system changes or updates have been made to either PC, other than the minecraft v1.5.0 update