Hi there,
There is a spec for input offset voltage, but there is also a spec for input offset current, which can be as high as 5ua.
5ua flowing though 20k will cause 0.1v of output offset, or as Carl suggests it can be partly due to bias current too.
The solution is to lower the feedback resistor value if your drive source can stand it and go from there, or better yet, choose a better op amp part as that op amp is an overkill on GBW for a 1kHz signal with gain only 2.5 or so.