Had exactly the same fault but found my solution (below). Happy to send you scan of correct process:
I see several buyers had issues with not being able to extinguish the Descaling lamp once they'd been through the descaling process. I had the same so back it went to Philips. Arrived back within a week with an extract from the user manual which stepped through a totally different (more automated) process from that shown in the manual that came with my Brera. Engineer's note: "NB on the new Brera, to start the descale programme switch the machine into standby mode, then press and hold the two coffee buttons for 6 seconds to start descaling".