I have emailed Stan last Friday, but until now I haven't had an answer from him so probably he is very busy.
The problem with no sound from the centre channel has happened again but this time the workaround with the test signals wasn't successful and I had to switch the SP3 off and on.
It looks like the issue arises after I have switched input. At the moment, only two inputs are in use: an analogue input for my streamer (audio-only via 2 channel by-pass) and a digital input for my Blu-ray player via HDMI. After I have been listening to music from my streamer my SP3 seems confused when I switch to my Blu-ray player.
Hopefully, there is a simple solution.