I've started getting this on a custom board with a BF527. It happens with both the HPUSB-ICE and the 100B emulators. I've tried deleting the sessions, and swapping USB port (which has solved problems in the past but not this time). The configurator test utility gives lots of green ticks every time. What does this error actually mean? It gives no hint as what to do to fix it.
The board is functional: it runs code from flash, but I can't get the debugger to take control of it.