pthung

Thanks Phil, that"s probably it. There's only one pedal in my ext chain so I used a mono to mono jack there. Same as pedal out to send/return R. Will try your idea (though I kinda solved it by flipping the pedal so I can use send/return L again)

I'm having a similar issue. Due to scarce pedalboard-real estate I changed the return of my external pedal from send/return L to send/return R. Not using 4CM and set-up worked fine with send/return L. Changed the block to FX Loop Right and...no signal. Does this mean that R is deactivated if nothing's connected to L (mono) ?