I have analysed the problem in more detail and have noticed the following:
NPrinting Designer saves the XLS file at startup under "C:\Users\user1\AppData\Local\Temp\954629fe154b4b8f81535131c746bd0a".
When creating a preview, this file is not overwritten, but a new file is created under "C:\Users\user1\Documents". This means that the file in the temp directory is no longer up-to-date. But the preview shows the "old" file.
When saving in NPrinting, however, the "old" XLS file is sent to the server.
Does anyone have an idea? Unfortunately I don't have the possibility to change any paths in NPrinting-Designer.
Update1: This error does not occur when Word reports are generated. In difference to Excel reports, Word reports are not stored under Documents.
Update2: In difference to Word-reports the Path of the Excel-Reports changes after starting the preview. In VBA the path (Thisworkbook.Path) of Excel-Reports change as described above. By previewing Word-Reports, there is no change of "Thisdocument.Path".