So I am able to reproduce this problem. The issue has to do with the fact that the the rain sensor checking is not bypassing master station when it should. The work around is as follows:
1) go to Edit Options and disable Master (i.e. select Master as None)
2) Now the master station becomes a normal station, you can edit its attribute and turn on the ‘Ignore Rain’ flag.
3) go back to Edit Options and enable Master (i.e. select a Master station)
This should fix the issue for now. We will address this in the next firmware 2.1.5. Thanks.