Explain me, please, what "loop response" is, to help me translate this term into other language
My interpretation is as follows: Because the term "loop response" was mentioned together with an AC analysis, I think this is nothing else than simulating the ac transfer function of a complete feedback system under open loop conditions - that means: LOOP GAIN.
Such a loop gain analysis is very important to get a picture on the closed-loop behaviour of the system (stability, stability margin, bandwidth, step response...).

EDIT: I think, this interpretation is supported by the fact that oscillations of the closed-loop system were observed.