Weird crossfader/channel fader issue (using my DDJ-RR)

So, I'm using an oldish Macbook Pro with a DDJ-RR.  Having updated to the latest Rekordbox version this evening (may or may not be related....who knows), see, to have had a load of weirdness going on.

Firstly of all, using the "find similar tracks" (can't think what's it called) on the right hand side of the screen, it started to constantly refresh and flicker as it was doing so - ended up turning that off as it was jsut not working.

Next I was getting a popup about only being able to tag 100 tracks or something, even though I'd not tagged anything. It was literally every time I tried to scroll and select a track. Resolved that by checking the relevant boxes in the settings about clearing the tagged tracks on exit, etc.

Then the sync button on the right side deck seemed to not be doing anything (having worked just fine for about an hour or so).  During the process of trying to resolve the below issue, this seems to have righted itself somehow, even though I've not consciously changed any settings or anything to have achieved this.

Then onto the main event - something seems to have gone utterly screwy with the crossfader/right deck channel fader.

The crossfader is completely unresponsive, yet if I move the right deck channel fader up or down, the cross fader also moves left/right.  Moving the channel fader up to full moves the cross fader to the right. Channel on minimum and the crossfader is on the left.

Fearing my trusty DDJ-RR had may have finally crapped out, I installed Rekordbox on a different macbook, to try determine if it was a hardware or software thing.  Everything works just fine on that machine thankfully, so it must be a software issue.

I've gone back to my original mac and scoured the settings a few times, and can't seem to find anything that would be causing this.  The 1,2,3,4 buttons on screen are all set as they should be for the crossfader (1,3 and 2,4 respectively). I've tried resetting all controller settings to defaults, made no difference.

I'm at a bit of a loss, so would welcome any suggestions, as I don't really want to have to uninstall Rekordbox and start over.

It does seem mighty coincidental that I've not had any issues whatsoever until it updated to 6.7.6 this evening.