This is ridiculous, how I try to do something for an hour, post the question and then get the solution myself. This link shows the list of parameters, but as I'm new to the Jasper, I make stupid mistakes.
https://community.jaspersoft.com/wiki/xls-export-parameters-jasperreports-server
I've tried to put the "ignore cell border" property this way:
<property name="ignoreCellBorder" value="true"/>
I don't know why the examples are written this way under the above link, but when I put this:
<property name="net.sf.jasperreports.export.xls.ignore.cell.border" value="false"/>
It started working.
