Hi JS S,
Have you tried the solution described above?
It is described here as well.
XML file validation failed with XSD and e0400b
Basically, I have opened the .p4d file using a text editor (notepad ++)
and I replaced the Optimizer type from _Classic _to _Ceres _as you will see in the screenshot below.
If it does not do the trick, you can download the Pix4Dmapper 3.3.29 here.
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Best,