I have modified the startup code slightly to avoid problems with the combobox not being filled (Update will probably be uploaded tomorrow).
I wonder what the bluetooth ports do, they may delay the serial part of the code a bit, i.e. it will take longer before the background threads reports ready. The screen shoot looks that way, it has found the meter, but not enabled the full user interface yet, this means it is still waiting for the background threads to be finished with all the ports.
If "Scan serial ports" is checked, it will start a thread for each serial port. This thread will do a *idn? and wait some time for an answer, before it quits. When it is not checked it will only start threads for ports where you have specified that there is a device.