/usr/lib/x86_64-linux-gnu/libthreadutil.so.6 is in libupnp6 1:1.6.19+git20160116-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libthreadutil.so.6.0.4
/usr/lib/x86_64-linux-gnu/libthreadutil.so.6.0.4