I also have this issue. Driving me nuts, trying so many different things to solve, LOL. I think it has something to do with the version of MS Offfice - there being a bug for (what I am using) MS Office 2010. What version of MS Office are you using?
I've had this problem when linking to Excel named cells for as long as I have worked with PDM. My guess is that the variable type for Inicio and Fin are date types. If this is the case, my workaround for that has been changing the PDM variable on the card to a text type.
Whatever the reason is, it does not like the date format.