Thank you all for pointing me into the right direction:
The problem is solved with the help of the programmer of the DotNet Zip Library.
"By default, Zip entries created by DotNetZip will include extra data, for the timestamp on the file. This data will appear in the zip file immediately after the filename, and immediately before the file content. Normally this is tolerated by zip tools that read the files. In some cases it is not. To turn off the timestamp, set EmitTimesInWindowsFormatWhenSaving to false, on the ZipEntry. This would be (e.EmitTimesInWindowsFormatWhenSaving = false
"
That will eliminate the extra data between the filename and the file content.
"
It worked and the .epub is validated OK ...
Thank you all very much for helping me solve the puzzle !
Martin