I have a Nov 2017 dsl 40 cr with pedl-90012 Footswitch. The effect loop is intermittently not foot switchable on the clean channel. I use a clean boost for solos through the loop. Works on dirty channel, not on clean channel. Footswitch effects loop led lights up,, but not on amp. Both green channel leds. I have the individual master volumes leds set for clean channel green, dirty channel red. And I have to push the loop button in and out most of the time to activate it.
For the firmware update circuit board swap.
I was told it's no longer a warranty issue. They sent it to me free of charge. Invoice said $51, but they charged it to some billing code of theirs. Seems to work OK now, but haven't gigged with it yet. It's a small board about the size of a business card. Sits vertically in roughly the middle of the main board. One small screw, one multi pin cable as well as multi pin connection on the board itself. See this video on YouTube with the title
"Inside the Marshall DSL 40 CR- Chassis Pull"