The SX2 was never a hardware unlock device for Rekordbox. It was designed for Serato and released well before Rekordbox released performance mode. Rekordbox did provide SX2 support with purchase of a license; I'm still happily using Rekordbox V5 and my SX2 with the license I purchased. However, I'm not interested in paying a monthly subscription so I haven't made the switch to V6.
Have you made any changes to your Rekordbox config, eg downloaded V6?