Try to define relationship with fluent api this way (works for me):
modelBuilder.Entity<Member>().HasKey(x => x.Id);
modelBuilder.Entity<Member>().HasOptional(x => x.Witness)
.WithMany()
.HasForeignKey(m => m.WitnessId);
modelBuilder.Entity<Member>().HasOptional(x => x.Reference)
.WithMany()
.HasForeignKey(m => m.ReferenceId);