The order of your filter is too high for the design type with a cutoff frequency that low, so you get numerical problems. To make a quick fix, reduce the order of your Butterworth filter to something a little lower than 10. An 8th order filer works for your design, or raising the cutoff frequency also results in real valued output.
If you want a filter with the frequency response specified by your design, you need to cascade lower order sections. See a detailed old answer I made for information on how to go about this. The tf2sos
function is your friend here.