Hi Thomas,
There is no libz.so file in the 1.8.6 distribution of PixInsight for Linux. You are probably mixing files from different versions when you install PixInsight.
When you uncompress the installation archive, please make sure that you remove any existing 'PixInsight' folder on your '/home/starhopper/pi-install/' directory before running the tar command. PixInsight 1.8.6 runs out-of-the box without problems on all tested Linux distributions.