Anders, where do you live? I'm away in Brazil at the moment, but I'd be happy to let you mail one of your PICs to me and I could fit it to my Minimig and reflash it for you. If I can reflash it then the Minimig board may be the problem, if I can't then it might be the PCs... or something you're (not) doing.
I'll be home in around 3 weeks.
Edit: If it does turn out to be the board then I'm sure ACube will fix it/replace it. Their service & support has been top-notch so far.
Out of interest, do your PCs have "real" serial ports or USB-Serial dongles?