Thanks for all the good answers.
I want to share the solution with you.

The actual solution of the problem was (given by Lars Appel):

"One potential issue that comes to mind
is that MPE/iX tar add's MPE specific file attributes in some kind
of Posix compatible way (extra /tmp/Xattributes.N files). If your Unix
machine does not like those, you might try adding -A when creating
the tar archive (to drop these MPE attribute files)."

So that all I had to do is
"tar -cvfA mytar thefile"
and ftp and untar worked fine!

Noam