I'm in the process of completely migrating over to rekordbox and certain files have the "unsupported" warning when adding to the rekordbox library.
They are all 44.1k 16 bit AIFF files (Not AIFF-C) I've checked and double checked this. I've even used a file converter to reconvert into AIFF and I still get this warning.
Bit more context: When viewing these folder(s) in the browser, the files that generate this warning do not display track title and artist columns in rekordbox BEFORE importing to the library, where as everything else does.
All of these files either came from bandcamp or beatport (as AIFF) or were converted from a wav from some other source.
The file conversion software used is switch. I honestly don't remember if these were converted wavs but I can say that many, many tracks in my library are converted wavs with switch. I've converted my entire library over to AIFF as a part of this process.