AAC files with the .aac extension currently have seeking issues, and exhibit pops and clicks when played back in Serato DJ Pro & Serato DJ Lite on Windows OSes - and will show an 'unsupported file type' message in the status bar when loaded to deck.

This does not extend to AAC files with the .m4a, .mp4, or any other extension, and solely affects .aac files.

While we further investigate this issue, as a work-around, we suggest you try re-encoding your affected files to file type with a different extension.
You can either use iTunes with our guide here, or use a third-party tool like Audacity.