The XML generated from MPP (using last version of MS Project) cames with empty section <extendedattributes> directly inside the Project tag.</extendedattributes>
If we look the XML generated by MS Project for the same gantt, it has 80 lines between <extendedattributes> and </extendedattributes> tags, corresponding to the extended attribute which we configured in MS Project, plus 10 blocks with the following form (Duration1 to Duration10):
<ExtendedAttribute>
<FieldID>188743783</FieldID>
<FieldName>Duration1</FieldName>
<Guid>000039B7-8BBE-4CEB-82C4-FA8C0B400067</Guid>
<SecondaryPID>255868968</SecondaryPID>
<SecondaryGuid>000039B7-8BBE-4CEB-82C4-FA8C0F404028</SecondaryGuid>
</ExtendedAttribute>
which we didn't configure.
Also, the XML from MPXJ has no <extendedattribute> tag inside <task>, while it do comes in the XML generated with MS Project.</task></extendedattribute>
There's no problem when we try with any MPP generated with older versions of MS Project.
It may be related to ticket 297 (https://sourceforge.net/p/mpxj/bugs/297) that we created in 2016.
We're using the 7.9.3 version.
Forget about it, sorry. The situation occurs with a specific file. It must be an error with MS Project.
If we create a new file with extended attrbutes, it converts flawlessly.