So, after upgrade to Thread SDK 0.11, I'm not able to perform DFU again. Symptoms are identical to devzone.nordicsemi.com/.../ - this time on nrfutil v3.4, with pyspinel==1.0.0a1 and 1.0.0a2. Linux desktop this time. Tried nrfutil 3.3.2 as well.
Board with the client is able to send trigger packet request to NCP/DFU server board, nrfutil sees the request, prints message about sending the reply, but reply never reaches client.
They seem to be in the same network (ping works, router table shows both devices, etc.).
Any other ideas why nrfutil->client communication might be faulty?