Hey dude. I just created this account so I could respond to this question. I had to go thru with it today and just figured it out.
When the center channel level lights are flashing on the ddj-400, the controller is in update mode. It is expecting a computer to transfer firmware to it via USB. The only way to bail out of this mode appears to be by giving it new firmware (rebooting/unplugging does not work).
So just update the firmware to fix. I followed these instructions here (starting at step c) https://www.pioneerdj.com/-/media/pioneerdj/downloads/firmwares/controllers/ddj-400/ddj-400_update_manual_e_1st.pdf. I uploaded firmware version 1.01, which is currently the newest available. Hope this helps!