Magento 2 APS Package and Extensions

Whenever I try to install an extension inside my magento 2.1 installation on plesk, my server crashes and I have to reinstall the whole application. When installing vie Setup Wizard, it is stuck at "Update pending" and a manual installation via console leads to the server being not reachable (HTTP Error 500). What can I do to install extensions properly?

EDIT: My System - Ubuntu 14.04, Plesk 12
When Update is pending, no errors appear in the log file and I cannot access magento since the...

Magento 2 APS Package and Extensions