Function that is different in Excel
Some Excel functions have known issues where they give the wrong answer in certain rare circumstances.
One particular example is XIRR, which gives #NUM if there is no negative cashflow in the first period. That does not mean that there is no internal rate of return for the series of cashflows.
The preview window in Openbox uses an Excel simulator, rather than Excel itself, to calculate, and that simulator’s XIRR function gives a correct answer in the case where there is no negative cashflow in the first period. So in such cases, you may see a (correct) value in the preview, but when you build the model in Excel, you will see #NUM!
The solution here is to use the same workaround for XIRR as you would in an Excel model, e.g. force a small negative cashflow in the first period.