I know this probably doesn't help much now, but I notice when I use too much water (while smoothing, in a spray bottle or spatula, etc) my bc cracks. I've also heard that using an all shortening icing cracks more than one that has part butter. hth
See if you can get another layer of something under the cake boards. If moving it causes the icing to crack, then the board isn't stiff enough.
If you have any large corrugated cardboard pieces, you might be able to cut another cake board, with the corrugations running the other direction (or even two more, each in a slightly different orientation) to keep the board from flexing under your cake.
Good luck!
Laura.
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