Did you change the default values or emptied the field for whatever skill you where rolling? I've encountered the same thing when the calculation if the roll is incorrect like having two plus-signs after eachother, stemming from the fact that I had accidentally emptied some fields despite the fact that they should have contained at least a zero to function. Is the roll using a template or the default one? If so, hover you mouse over the zero and look at the math that it shows and look if there is something weird there.
