Was trying to mess with the DXL Importer, and
came across that the reason for the error message
DXL importer operation failed error
was because the the code in a script library was not valid.
Upon saving the code in the scriptlibrary, a compile error occured, solving this error, the
DXL Exporter --> DXL Importer ran smoothly.
I guess that the DXL Importer tries to compile the code before saving it into the designdocuments, and thus the error.
So go through all design elements and try to resave them to locate errors. There could be som Notes application developed on R5 that have function calls that are conflicting with new internal LS functions.