Gromacs: What can be reason of getting final potential energy greater than the starting one?
You are right, in a normal simulation using conjugate gradient (i.e. an energy minimization) the total energy should gradually decrease towards a plateau.

Based on the numbers in the log I guess that there are some atom clashes. The potential energy of the system is too high and the forces also. I would check atom 54660 (as indicated in the log) to see if you have it too close to other atoms or in a strange configuration. If you don't see anything wrong, double check the dimension of the simulation box, maybe it is too small and it causes problem with the periodic boundary conditions.

Anyway, energy minimization using conjugate gradients (CG) should be able to solve these kind of problems. You say you run gromacs in single precision. Try to use gromacs compiled in double precision, at least at the beginning of the energy minimization. If the initial conformation of the system is very bad, single precision can be unstable and unable to minimize the energy.