I forgot to mention about tuning of f_imc; after leaving PSL room we found that the peak-to-peak value of the error signal of IMC got much smaller than the nominal value. We thought that due to change of f_imc not only AM sideband but also PM sideband got smaller. Actually, after changing f_imc back to the original value, 13.77 MHz, the error signal seemed proper, and with this modulation frequency we could lock IMC.