Why am I getting 10V at the opamp output instead of 0v?
Your op-amp is driving the +5V rail out of spec through the protection diodes in the ADC. Not a great design, but it should not normally happen. 

However, many 4-20mA inputs use 22mA or more to indicate a broken sensor etc. so it's not uncommon that the op-amp might produce more than +5V. Usually a good designer would provide a clamp to keep it from causing problems, particularly when you are sharing supply rails with other channels.  

The op-amp output should not be at +10V unless your jumper P3 is missing. It should be at about 0V with no input. 

As a quick fix, assuming it's just the jumper missing that is causing the op-amp to rail, I would be tempted to reduce the sense resistor to perhaps 150 ohms and give up a bit of resolution.