Since version 14.9 projects will not open with my 2nd monitor plugged in. The clock and mac's spinning pinwheel will spin forever until I force quit Premiere. I always have to unplug my 2nd monitor's HDMI cable in order to open a project. When it is open, I can switch mercury transmit off and on again and the 2nd monitor works fine. So I found my workaround, but it is pain.
Some diagnostic questions:
Mac or Windows?
Do you have any 3rd party Transmit plug-ins installed?
You mention 14.9; do you experience the same behavior, in 15.1?