Trying to import a database with 329,866 entries fails

My customer has send me a database with 329,866 entries.  I have got an XML export and the corresponding xdt. Creating a TB is not a problem. But I cannot import the TB, after importing it in various ways I have 0 entries and a log file, telling me, that entries up to 102,494 has been added. From this entry up to the end of the database it says, that the entry does not fit the termbase definition (this can be excluded, as the XML does fit the TB definition for sure). There is a reason given: Database ´´ could not be opened. Either the database is not recognized by your application or the file is damaged. Source: MultiTerm140.Server.Termbase14.0

Unfortunately I am not allowed to pass the xml or the xdt to anyone. But maybe you ca give me a hint what to do based on the error messages I posted.