LAN bug, cannot connect any lan

I cant join mu cousins LAN world, hes on MAC i dont know i its affect that but its reeeaaly annoying because i cant join any lan worlds from each others, we are connected to each other (we tested with a program) but we cant connect to each other

we can see the lan world but when we try to join its telling Connecting to server... and then connection failed, we tried direct connect but no avail

i think its affected by the new launcher, because playing on 1.5.1 would not work too

please fix this bug!

P.S. sorry for my bad english, im dutch