I dont see how it could be the lights unless they are over heating the cabinet. No way is 56 watts total power too much for 4 sq ft - especially at that distance.
There must be something else going on when you put them in there.
What is the temp in the cabinet when the lights are on?
Do you have any before and after pics? That might help diagnose the issue.
Also, any other details would help - soil or hydro? Nutes? PH? EC?