Hmm. That is perplexing. You might start a new PivotTable from scratch and try to rebuild it to match exactly. Unfortunately Excel doesn't offer up very good error messages, so it's hard to know the problem.
Also, on the existing PivotTable which is causing problems, I would go to the MDX tab in OLAP PivotTable Extensions, copy that MDX, then send it to your OLAP administrator to troubleshoot what's going wrong.
Are there any interdependencies between the calculations you've added via OLAP PivotTable Extensions? I wonder if the sequence you add them matters.
Not sure how else to suggest troubleshooting the problem.