When uploading audio files for a promo, if the watermarking pre-processor encounters an error, it will highlight the suspect track in red.

MP3 files can get corrupted if there is an issue while encoding them or interference while uploading to our servers.

Look at the suspect track's properties and confirm the bitrate stays set to what you initially encoded it to be.  Try re-encoding the track and uploading it again.  If it's a variable bitrate, changing it to constant bitrate can sometimes help too.