When decurling 0.028” thick (500 gsm) coated recycled board, the surface of the sheet gets cracked. What can I do?

You might consider ordering your board “coated side in”, so that the web feeds into the decurl section with coated side down. If this is not practical, investigate ordering your roll stock on larger diameter cores. (Most of the cracking occurs at the end of the roll where more decurl is needed.) Cores diameters of up to 16″ (400 mm) are used on heavy board to minimize the curl set.