If I generate an excel-report, excel 2010 shows the following warning message:
file error: data may have been lost
Note: I already have found the solution and will post it immediately. I make this entry for other people having the same error.
It turned out, that the reports data source had in a value of -0 (negative zero). The datatype was decimal. Excel can not handle this.
The problem seems to lie in the Excel formula engine and not in the report renderer (However I think MS has to solve the problem in the report renderer).