A few things could be causing this....

Firstly your code:

CASE WHEN [Measures].[R]= 0 THEN 0

This returns a zero rather than a NULL so any NON EMPTY precedents in the queries (these are there by default in the cube browser and Excel) will not work as expected. This may be causing your issue.

Secondly the form of your code is not considered the best in terms of performance. Try:

IIF( [Measures].[R]=0, NULL, [Measures]./[Measures].[R] )

The reasons for this are discussed here: http://sqlblog.com/blogs/mosha/archive/2007/01/28/performance-of-iif-function-in-mdx.aspx