This error started appearing with v1.0.8 (still present with v1.0.9), preventing wireproxy from starting. v1.0.7 works correctly.

Seems to have been introduced by a679716

I have confirmed that /dev/null is as should be:
crw-r--r-- 1 root root 1, 3 Apr 23 21:08 /dev/null

and have recreated it (just in case):

# rm /dev/null && mknod /dev/null c 1 3 && chmod 666 /dev/null && /opt/wireproxy/wireproxy -d -c /opt/wireproxy/wireproxy.cfg open /dev/null: permission denied