The pawn fork is the ultimate problem. 1.d4 Nf6 2.Nf3 g6 3.Bf4 Bg7 4.e3 d6 5.h3 O-O 6.Be2 Nbd7 7.O-O Qe8 and now 8...e5 is coming. If the Bishop is on d3, you have problems with the pawn fork on e4 and also the d7-Knight going to c5 to gain a tempo on the Bishop.
The Bishop should go to e2.
on d3 the Bishop is useless as the diagonal is blocked by the g6 pawn and it's harmful as it blocks the Queen's vision on the d-file while on e2 it doesn't hurt your coordination and is more effective. I don't play the London but this is a general principle in Queen's Pawn openings
In my humble experience, although d3 looks bad, it helps develop pawn to e4 with d2 knight backing up. Especially when opp plays d5..
I would like to ask experienced players about the placement of the LSB in the London system against King's Indian and fianchetto set ups - why in these sorts of positions the LSB is placed on e2, as opposed to how it normally is placed on d3? What is the critical difference between these two different moves, and how they correspond with white's ideas against fianchetto and other systems respectively?