Pete,
from what you write about having recently replaced the centreboard uphaul pulley blocks, I think you have by accident also changed the max-down angle of the centreboard, so that now the centreboard can swing too far forward, and can then catch the underside of the two hardwood cheeks on either inside of the case when being hauled. This can have very serious consequences as the hardwood cheeks are vital to hold and guide the board, to take the lateral strain.
Do look in and check, and to remedy, limit the "full down" of the board so that the board cannot escape from between the two cheeks.
C.