I haven't had time to play with this any further, and it was working perfectly since my last post (11 days ago). During that time I have made no changes to the settings of MWB, nor any changes to Java or Unifi. The machine has been rebooted for some Windows updates (with no changes to MWB settings), and Nvidia updates, yet Unifi and MWB continued to work in perfect harmony. Throughout that time MWB was on, with inbound protection and exploit protection both ON.
Then, with no changes to anything, MWB suddenly this afternoon popped up the "generic" error (as above) and killed Unifi stone dead. Out of the blue! 11 days it worked, including reboots, and then decided to revert to its original behaviour. As described previously, the procedure of switching off Java incoming, disabling exploit protection, killing MWB and then restarting Unifi allowed it to work. Then, as before, I started MWB, re-enabled exploit protection and finally Java incoming, and everything is happy again!
There's something really odd about this v4 MWB.