Well my first guess would be that it has something to do with the cell style chosen in your excel export template. The error logs things about the color of it.
Do you have any alternative colors/styles etc in one of the cells of the template?
Which version of the Excel Exporter do you use?
Which version of poi is in your userlib folder?
Hello,
By deleting older version of each library that was doubled, it finally worked.
Also, the only excel format that works for me is the Excel 97-2003.