Oracle Primavera P6 17.7 has a known bug that results in PMXML files not getting imported successfully.
The problem is that the import library is assuming that a valid Calendar ID is present for every task in the xml, and it does not invoke the code correctly to use the default Calendar if a task has no calendar associated with it.
The error will look something like below:
Primavera has openly reported a similar issue with importing MPP XML files as well.
The details can be found in this link:
There are other related errors logged related to import of XML files with P6 18.8 versions as well. Two main issues are described in the following links:
The import works in some versions of P6 and not in others. The operating system is also a factor in some cases as seen in the blog in the link below:
Please note that the primavera major and minor versions have to be checked before using it to import xml files.