Just in case other folks are having issues.
Something I have noticed with ENVI 5.5 is that XML files do not appear to be read or ingested by ENVI processes (QUAC) if the xml file exists deep within a file folder structure.
For example, I am working with Pleiades imagery. Once copied onto the harddrive, you have to drill deep into a sub-folder structure with very long naming conventions to access the multispectral and panchromatic xml dimap tif files. If you input the xml into the QUAC or simply try to open in the viewer, the data does not display if the required files are buried deep within the sub-folder file structure. To resolve the issue, I have simply copy/pasted the data higher into the folder structure. Thus, QUAC and ENVI easily ingest the data for processing.

This was not an issue in ENVI 5.4.

AerialFrank