This is probably caused by some sort of MDX failure/incompatibility/Excel read error.

The cell that is meant to be showing a value:

1. Is is a calculated measure?

2. Does it have a FORMAT_STRING specified (if it is a calculated member or not).

3. What should it display (a number, a string, a percentage, etc.)?

4. If it is a string value then does it have an excel reserved character (=, &, etc.)?

You really need to provide a little more information to get a good answer 🙂

Edit: Martin and I posted at the same(ish) time. So add what he said to my list of things to check. If that is the issue and you are on 2012 or later then you can use the DIVIDE keyword in your calculation instead of the / operator. In earlier versions you will have to wrap the calculation in an IIF logic.