Planmaker xlsx files not recognized as proper excel when importing into other software

[avsu]

When trying to import a Planmaker NX file saved as XLSX file into another program it will not import and gives the error wrong file format FLEXCEL.

If I open that file in Excel and just do a save and then try to import it into the other program then it imports flawless.
It seems to me that it may have to do with the file headers of the XLSX file generated by Planmaker NX that causes the other program to not recognise it as a proper Excel file.

Is this a known issue that may happen with other programs or is it a bug in Planmaker NX itself?