Could you show your input? I usually get this error if I forget to set scf_type = pk. By default, I believe that scf_type = DF and this will not create the conventional integrals causing this error with iwl_buf_init.
It would be helpful to see your input file. Although, in principle, one can use any orbitals for DCFT as a guess, we do not currently support using DF-SCF to generate guess orbitals/density for DCFT. It would make sense to do that for density-fitted DCFT, which soon will be released.