Have you checked the acces rules of your file document entity? I had the same error when not setting up the rights to the file document correctly. Also, see https://forum.mendix.com/link/questions/95757.
Can you show the microflow which is used to produce the Excel Export?
If the access rules are not the reason for the problem (as Casper said), then it might be that you have configured an input object in you rexcel export template which is not passed to the the Excel Export Java Action
It seems to be an access right topic. :-) With the security turned off, everthing works fine. I have to figure out, how I can change the access rights to the FileDocument-Entity. I was never in this part of Mendix and everything is greyed out. ;-)