OK, i've downloaded the 1.5.5. files from GitHub and uploaded them. Still getting blank Admin:

Code:
[01-Nov-2016 14:40:10 UTC] PHP Fatal error:  Uncaught Error: Call to undefined function zen_get_ip_address() in /var/www/vhosts/localhost.localdomain/httpdocs/IQ/shop/ADMIN/includes/init_includes/init_sessions.php:31
Stack trace:
#0 /var/www/vhosts/localhost.localdomain/httpdocs/IQ/shop/includes/autoload_func.php(48): require()
#1 /var/www/vhosts/localhost.localdomain/httpdocs/IQ/shop/ADMIN/includes/application_top.php(171): require('/var/www/vhosts...')
#2 /var/www/vhosts/localhost.localdomain/httpdocs/IQ/shop/ADMIN/index.php(10): require('/var/www/vhosts...')
#3 {main}
  thrown in /var/www/vhosts/localhost.localdomain/httpdocs/IQ/shop/ADMIN/includes/init_includes/init_sessions.php on line 31
My 1.5.5a ZenCart uses a database that I imported from an old 1.3.8 installation where I used to use the old version of Stocks By Attributes. Could this be a cause for any of the issues I am having here?

Thanks

Dene