Need help with instrumentation amp behaviour
The problem is you are not applying voltage to the inputs, which makes them 'float'. Instrumentation amplifiers have a very low input bias current in the pico volt range normally (meaning it takes pico volts for them to make a measurement). A lot of times this is close to currents flowing through the air. 

Odds are there is not a lot of resistance through the caps and you are seeing the leakage through them.

Apply some voltage to the inputs (or in your case the resistors before the inputs) and the problem should go away.

Edit: Yep, the INA155 has a very low input bias current:


  LOW INPUT BIAS CURRENT: 0.2pA


If you didn't have the filter could wave your hand over the inputs and the value of the amplifier would change from charge of your hand. 

Tie the inputs to something that isn't floating like ground or a voltage reverence. (A 1.5V battery is sometimes fun for a voltage source)
I forgot you had a gain of ten, you'd have to divide down your 1.5V battery with a 10 to 1 resistor attenuator.