rekordbox froze when I canceled DJM-250MK2 firmware update. Click [Cancel] or [Close] on Updater Program launched from rekordbox.

You can solve the issue by launching the DJM-250MK2 in normal mode.
Please follow the steps below.

Steps
(1) Close rekordbox.
　Windows:
1. Press [Ctrl]+[Alt]+[Delete] and click [Task Manager].
2. Click [Processes] tub in Task Manager.
3. Window10 and 8.1: Click "rekordbox program file" and [End task].
Window7: Click "rekordbox.exe" and [End process].
→ rekordbox closes.
4. Click [X] button in Task Manager to close it.

　Mac:
1. Press [command]+[option]+[esc] and click "rekordbox" in "Force Quit Applications" window.
2. Click [Force Quit].
3. ["Do you want to force "rekordbox" to quit?] message appears. Click [Force Quit].
→rekordbox closes.
4. Click [X] button in "Force Quit Applications" window to close it.

(2) Turn off the power of the DJM-250MK2, then turn it on again.
(3) Launch rekordbox.