Forget an Upgrade, install a new virgin OC v.1.5.6.5 Software,
by use of a new Database, and if it works as planned, you could
always change the link to your older Version Database, to find out,
if it still works, or what would eventually has to be modified in your
old DB, to make the system work again.
And when it comes to already existing Extensions, you need to
install ONE after ONE, and then check in the Admin Error Section,
to so find Errors, related to that Extension. Front-Side-related
Extensions will most likely show errors, by Use of a 'Responsive'
Theme, because those Themes require partly different Code as
well, to function again.
The Opencart 'upgrade-Code' and 'Info' should be removed from all
OC Versions anyway, it's, in most cases, just a waste of time, playing
around with such. Most all OpenCart Versions are modified in some Parts,
due to adding Extensions, and no default-only OC update-routine would
be able, to handle such. This Forum is full of pointless postings,
just because of that.
Ernie