So a screen shot - NFB loop is open.
Yellow: at load resistance
Blue: one leg of the PI through a coupling capacitor.
The resonance is caused by the transformer, isn't it?
I would learn how to compensate for it. Damping on the primary of the PI? Using a a cap in series with the resistor in order not to affect the lower frequencies? How to estimate its values? (i recognize the frequency of the resonance can be read from the screen shots).
And i would like to understand how it is mapped backward into the circuit and whats the reason for the doubled frequency?
and the schematics: