The file you tried to upload isn't a valid BibTeXML File. The DTD specifies, that the concrete entry (e.g. article) must be defined after the entry-tag. This is missing in your XML example. Please correct this and try again

<!ELEMENT bibtex:entry ( bibtex:article | bibtex:book | bibtex:booklet |
bibtex:manual | bibtex:techreport |
bibtex:mastersthesis | bibtex:phdthesis |
bibtex:inbook | bibtex:incollection |
bibtex:proceedings | bibtex:inproceedings |
bibtex:conference |
bibtex:unpublished | bibtex:misc ) >

There is a DTD file? Nice ^-^ ... Where can I find it?

Also I guess the bibtexml and the bibtex might have a few differences in naming conventions, I'll have to check it but I guess this is a matter of uppercase / lowercase characters or maybe one type that has to be spelled differently.

The exported xml is now correctly imported in NESPOS, NESPOS can also re-export the importation without information loss or field confusion.

The problem is that NESPOS does not seem to allow editing the fields of the freshly imported bibliographic references.

This is a bug for NESPOS and not for MARS tough. So this issue counts as solved.