The physical conditions of the rise and development of the instability of the second harmonics of the electron modified Bernstein modes in a plasma of a loop in the arch structure are investigated at the early stages of the flare process in solar active region. The wave perturbations rising at the frequency close to the frequency of the second harmonics of the electron cyclotron waves propagate in the direction which is almost perpendicular to the direction of the magnetic field of a loop. The development of instability is the result of the collective action of the effects connected with taking into account the pair Coulomb collisions and the existence of a weak «subdreicer» electric field with slowly increasing amplitude in the current circuit of a loop. In the framework of Heyvaerts-Priest-Rust theory of a flare, the growth of the amplitude is caused by the increase of interaction of the magnetic fluxes during the flare process. On the background of the turbulence developed from the instability of the first harmonics, the instability of the second one can rise only at the limit of the used approximations. It is shown that on the background of the Bernstein mode's turbulence the appearance of ion-acoustic wave instability is possible in the subphotospheric layers of a loop in some specific interval of the heights. In the same situation the Langmuir wave instability can rise simultaneously with ion-acoustic one or before it according to the harmonics the instability of which produced the background turbulence.