There's a thread a few down the page about that error.
Shirik posted this to fix it:
Right click on the shortcut icon and choose "Properties." In the window that appears, look for a field that says "Working Directory" or "Start In" and fill it in with the location where Minion is installed (such as "C:\Program Files\MMOUI Minion")
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Hope that sorts it for you, but it is beta software so there are going to be hiccoughs at times!