High-voltage (HV) messing with my transimpedance amplifier circuit
All input and return signal paths must couple equally to stray noise to cancel. This is normally done with shielded twisted pairs and for lower capacitance on cable using a common mode signal to drive the shield over twisted pairs, called an Active guard method. 
Ensure the CM signal is within TIA input limits and better performance is obtained using a ferrite balun sleeve or SMT twin coil CM choke with differential cap LPF.
No open loops in sensor wires are permitted as this creates a loop antenna.


More details on your exact signal being measured, the interfering signal, and the physical layout are needed to give a better answer.