Opening the .pas and .dfm files does not involve XML parsing. The fact that your unit uses some XML related units also does not involve parsing of XML at unit load time.
So this sounds like the IDE has a problem opening some auxiliary file associated with the source file. And something about this file or its reference in the source has not survived the transition from XE4. Look for files with the same stem as the troublesome source unit, but a different extension. Or look for IDE generated {...}
comments in the source unit that refer to auxiliary files. This should hopefully lead you to a solution.