qbe report excel export error

Hi All,
I'm new in SpagoBI and I'm using SpagoBI 5.1 and I have a problem with the QBE or Ad-HOC Reporting.

I created an ad-hoc reporting with a calculated field, this field is the percentage different between 2 fields with sum of amount.
This different is in percentuage and some value is in negative. When I try export report in xls or xlsx I have an error, SpagoBI open new webpage with the message: file not found
So I don't download the report.
The problem is the negative values, but in a different it's possible find negative values.

I think is a bug of spagobi but how can risolve it? There is a mode to bypass the problem?

Thanks

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