Unfortunately this is a problem with thick seat tubes, that the derailer is too far outboard. I have had to modify the derailer for my Xootr Swift to just work but it is not nice. The only solution is to find a thick spacer and get the crankset even further outboard. This messes up the chainline also. A triple crankset fitted with double rings is slightly better but the chainline is still as bad.

One of those compromises with foldies.