Hi Keith,
haven't had any problem here with an earlier build.
I'm trying to replicate what your are doing:
1) Configure/Plugins/Modbus
2) Click Add NEW Modbus Connection icon top left
3) Name, Description and Type <Next>
4) Enter Serial Com properties, check 485 <Next>
5) Confirm <Finish> or <Cancel>
I have these three screenshots of 3),4),5) Do they match what you see?
Craig