Dear Igor. I has the same problem and few days a go Prof. Hulpke write to
"You have reached the limits of the capabilities of GAP 3 as documented in the manual. The subgroup lattice program in GAP 3 is only guaranteed to work if the groups solvable residuum is not larger than 5000. (The algorithm uses a data base of recomputed information and this information is not available for larger groups). Your group <g> is perfect an thus equal to its solvable residuum, so there is no easy way to compute the subgroup lattice. The only way around this problem would be to provide the pwerfect subgroups a priori from theory. The manual secion on `Lattice' gives an example on how to do this. The subgroup lattice function in GAP 4 has been improved and will work for much larger groups (and -- provided memory being available -- should work for all subgroups of S12). However the code is quite different and thus it is not possible to make this available for GAP 3."
I hope this help you.