Im having this exact sam issue right now, December 2024, I tried to update, and reinstall rekordbox but still is not connecting to my controller, and I've neem using this for the last 3 years with out a problem

 

"I have been using my DDJ-400 at home tonpracticebon for the last 4 months without issues. Last time I used was last night. Allmof a sudden today the controller won’t connect to rekordbox. I have tried updating both rekordbox and the controller, but still nothing. The lights come on when I plug it in, then they go dim and nothing happens when I try to use the functions. I can play through rekordbox by clicking the play button on the screen through my laptop but not when I try to use the controller."