Since upgrading to OS X Sierra 10.12, I've started using PropellerIDE. (Thanks, Brett, Steve, and Parallax!) Initially I had a problem with the FTDI serial ports, including but not limited to the Prop Plug and my own FT231X boards. I hadn't seen this issue mentioned here and it took some hair tearing and googling to figure it out. The issue is that Apple, starting with OS X Mavericks (10.9), has included a driver for FTDI serial ports along with the system install. If you then go and download install the FTDI version 2.3 VCP driver directly from FTDIchip.com, the two will conflict. The symptoms are that you will be able to use the port once, but then if you disconnect the dongle and then come back to it or try to use another one on the same USB port, you will find that it is locked. The only way to get another try is to move to another physical USB port until that gets locks and so on. If you look in the system report under USB, you will probably find that the same VCP serial port is listed twice. To fix this,
delete the FTDI driver. AN134 from FTDI
describes how. Delete it using Terminal from /Library/Extensions/FTDIUSBserialDriver.kext. Keep the Apple FTDI driver, which on Sierra is located at /System/Library/Extensions/AppleUSBFTDI.kext. The name and exact location of the Apple driver on pre-Sierra OS versions is different though.
I see that the instructions on the Parallax web site page for PropellerIDE specifically recommend downloading the FTDI version of the driver. That suggestion should probably be stricken or nuanced.