Hello Trent,
I am unsure why your environment would act differently.
sometimes it worked as expected, somtimes I got a cross-thread error.
I looked through the code to ensure that there was no cross-thread violations...and I did not see any.
Accessing the combobox from inside the DoWork-Method is causing the error. The combobox is an element instantiated outside the DoWork thread of the BackgroundWorker and therefore lies at another thread, I guess. I don't know why it is working sometimes, but I don't know at all why Windows is working sometimes, or not... 
However, we are about to rework this dialog anyway, so I will be sure to take this into account!
Thank you very much, this will be great! 
But could you localization issues also take into account? It would save me (and other international customers too) tons of work...
Thank you very much and friendly greetings,
Ralph