I plan to try the native excel import on a PC, I'm not finding any helpful resources for the Theta extension. Seemed like a great workaround for Mac but the error message is just so vague.
The restricted fields is an interesting theory, but it seems to be specifically referring to the field 'No.' short for number. I don't think that's a restricted field, and I'm really confused about what filter it's referring to!!
But anyway, I think using a PC would have saved me a few hours of work. At least until MS gets the add-in for excel compatible with Mac.