No audio from 'Output Deck2' to CDJ900 since upgrade to RB 4.2

Since the latest Rekordbox upgrade to 4.2.0 from 4.1.2 - I am no longer able to hear sound from my CDJ900 which has been assigned to 'Output Deck2' in Rekordbox and connected to channel 2 on my external mixer. 

If I change the audio settings in Rekordbox so that 'Output Deck1' is assigned to 'CDJ/XDJ (2)' and 'Output Deck2' is assigned to 'CDJ/XDJ1' then I am not able to hear sound from my other CDJ900 which has been assigned to 'Output Deck2'.

This was all working absolutely fine before the upgrade. I have uninstalled the software and re-installed the latest updated 4.2.0b and this still hasn't solved the problem. 

I have also tried going back to 4.1.2. but still getting the same problem. 

Very frustrating. Any ideas?