Op. Amp. read variation when Earth is connected with Signal GND
What are you doing with your resistor values? You need matching to obtain minimal common mode errors (such as when neutral becomes connected to a different point or ground): -



Study the above. R1 and R2 and R3 = R4 and R5 and R6. Even C1 = C2 and importantly R7 and R8 are equal.

If you got rid of your R12 (s/c) and made R1 and R9 each 16k4 (twice 8k2) then it would work better. You also need to ensure that your R11 and R14 are rated for the incoming supply voltage maximum peak voltage. That is why you see three resistors in series in the reference design I showed.