Powercube Red flashing light/will not arm

Hi There. My QQ190 is on BF 3.1.7 and was flying beautifully this morning. Since it last flew, all I have done is turned off the second D filter to test, motors were warm, turned the filter back on in BF PID/Filter settings (as it was when last flown successfully). Also increased the Yaw P gain by 1 unit. That is all. When I went out to fly after this, my quad would not arm and I have a constant flashing red light on the cube next to the blue and green LED’s. I re-bound to my Spektrum in case I had lost my bind. It re-bound successfully, but I am still left with the flashing red LED and an inability to Arm. If I connect to BF config, the receiver is recognised, as are the sticks and modes (including the arm switch), but no arm.

I’ve read the QQ manual and the powercube manual, but cannot find any reference to this problem – in fact, the red LED is not mentioend at all.

Any help here appreciated.

Regards & TIA

Jeremy