Does this crank have the correct BB?
IF the big ring is sitting too far in, the cage is already into its downward travel.
IF it cleared a 52T ring before, there has to be something screwed up to NOT fit a smaller 50T ring.
Forgot the FDER change.
Is there enough "meat" on the tab to lengthen the slot with a round file?
ELSE, put the old FDER back on.