Single supply amplification
OK, you have a gain of 50 and any difference between the voltage divider (R6 and R7) that feeds U2 and the voltage divider (R3 and R4) that feeds U1 will be amplified by 50 and produce the error.

FIX: - 

Get rid of R4 and R3 and connect R5 directly to the output of U2. Also put C2 across R7 to reduce noise.