joomla white page in template after server migration/update // comparing packages

I changed servers mounted all the stuff and some of the Joomla sites I had opened correctly, others just showed a white page.

I didn't know why.

After many many failed attemps to fix it, god's hand enlightened me and I decided to compare the installed php packages between servers.

This can be done using

dpkg --get-selections | grep php

Comparing these packages between servers, it showed evidently I had missed installation of the package

php5-mcrypt

An apt-get install put my joomla sites running again.