For anyone with Reaper using this as an instrument track, I had this error message suddenly appear out of the blue while I was editing a block. Woe is my stupidity! I think my fat ol’ digits managed to find some key combo that turned off SD3 in the track F/X screen (piccy attached). In the list down the side and the checkboxes next to the name of the effect, I’d managed to uncheck it. Checking it re-enabled SD3 and error is gone! You can probably do this with automation too.
