okay i have found the problem.
We had changed in last version the import process (for both jD series) so that the download IDs are still the same as in the old series.
But this works not with the Joomla 2.5 version. We had this not really tested this, since the support for Joomla 2.5 releases has ended by the joomla core team.
Maybe can i use the older jD version for the import...