Its probably the rack bolts.

They have nylstop nuts, so you can tighten them a little bit less without risk.

One easy solution to reduce the chainline that you could try is to place the chainring on the inner position instead of the outer position (the spider of the Birdy crankset is foreseen to place two chanrings, one outer and one inner, the single chainring is factory installed at the place of the outer chainring, placing the chaining on the inner position may require to remove the plastic chainring protection)