Power Profiler Kit Error - Lowlevel error: JLINKARM_DLL_ERROR (ffffff9a)

I'm trying to get the PPK working when connected to aNRF52840-DK, but getting the error  -Error while setting up device [REDACTED_SERIAL]: Error occured when close opened device. Errorcode: CouldNotOpenDevice (0x4) Lowlevel error: JLINKARM_DLL_ERROR (ffffff9a).

This only seems to happen when the PPK is connected to the DK - the DK works fine by itself (or seems to at least); the PPK works fine when I use it standalone and connect it through a different JLINK programmer; but when I try to use both together I get the error.

I've set up up as per the documentation (set all switches and cut the jumper), updated nRF Connect and updated to the latest JLINK drivers. I've tried downgrading both app and drivers, with no success there either. I've also tried erasing the DK and re-flashing the PPK but neither have had any effect.

Could anybody shed any light on this please? The only thread I can find that's similar (https://devzone.nordicsemi.com/f/nordic-q-a/37234/not-possible-to-use-ppk-while-debugging) suggests its a JLINK driver fault, but that was 11 months ago so surely that would have been fixed by now? I'm also not trying to debug or anything, I'm just trying to get a measurement.

The log I get is as follows (I've x'd out any personal information).

2019-08-07T19:23:57.339Z INFO Application data folder: /Users/XXXXXXXX/Library/Application Support/nrfconnect/pc-nrfconnect-ppk
2019-08-07T19:23:57.343Z INFO App initialized
2019-08-07T19:23:57.629Z DEBUG App pc-nrfconnect-ppk v2.2.0 official
2019-08-07T19:23:57.629Z DEBUG App path: /Users/XXXXXXXXXXX/.nrfconnect-apps/node_modules/pc-nrfconnect-ppk
2019-08-07T19:23:57.629Z DEBUG nRFConnect 3.0.0 is supported by the app (^3.0.0)
2019-08-07T19:23:57.629Z DEBUG nRFConnect path: /Applications/nRF Connect.app/Contents/Resources/app.asar
2019-08-07T19:23:57.629Z DEBUG HomeDir: /Users/XXXXXXXXXXX
2019-08-07T19:23:57.629Z DEBUG TmpDir: /var/folders/30/zvpj033s0b51hbn_29547t5c0000gn/T/
2019-08-07T19:23:57.630Z VERBOSE Could not fetch serial number for serial port at /dev/tty.Bluetooth-Incoming-Port
2019-08-07T19:24:00.708Z INFO Validating firmware for device with s/n XXXXXXXXXXXXX
2019-08-07T19:24:00.711Z INFO PPK closed
2019-08-07T19:24:02.554Z ERROR Error while setting up device XXXXXXXXXXXXXX: Error occured when close opened device. Errorcode: CouldNotOpenDevice (0x4)
Lowlevel error: JLINKARM_DLL_ERROR (ffffff9a)

2019-08-07T19:24:02.554Z INFO Deselecting device
2019-08-07T19:24:02.556Z INFO PPK closed

Thanks