failed to rename the admin folder (URGENT)

Hi fellows!
We studied on the web, and many sources advised us to change the default admin of opencart 1.5.6 to a new path for security reasons (such as avoiding hackers trying to guess password).
As such, yesterday we tried to change the admin folder from /admin to /xxxxxxxxxxx7, then modified 3 files /admin/config.php , /admin/view/stylesheet/adsmart_search.css and /admin/view/template/report/adv_customers.tpl to have the /admin text replaced with /xxxxxxxxxxx7
After the change, we can see the admin login screen at the new url [oursite].hk/xxxxxxxxxxx7
However if we try to login as admin, the normal admin screen does not show up, it just returns us to the same admin login screen
I wonder if you have a clue why this is not working…Could a restart of our host help the system to back to normal? We have already tried to undo the changes but it still doesn't work!
Also, now when someone tries to click the login link on our opencart homepage, the screen goes blank with the error message
Fatal error: Call to a member function get() on a non-object in /var/www/vhosts/[myhost].hk/PE/index.php on line 113
Thank you

now after reboot of server, we saw the system failing to start mysqld. we also noted we only have 10gb disk space. so apparently the low free space is causing the db failing to operate. a study of the log file also has repeated errors like:
2017-02-03 6:49:27 - PHP Notice: Undefined variable: banner in /var/www/vhosts/[our domain].hk/PE/vqmod/vqcache/vq2-admin_view_template_common_header.tpl on line 374
2017-02-03 6:49:27 - PHP Notice: Undefined variable: payment in /var/www/vhosts/[our domain].hk/PE/vqmod/vqcache/vq2-admin_view_template_common_header.tpl on line 380
2017-02-03 6:49:27 - PHP Notice: Undefined variable: shipping in /var/www/vhosts/[our domain].hk/PE/vqmod/vqcache/vq2-admin_view_template_common_header.tpl on line 386
2017-02-03 6:49:27 - PHP Notice: Undefined variable: contact in /var/www/vhosts/[our domain].hk/PE/vqmod/vqcache/vq2-admin_view_template_common_header.tpl on line 392
2017-02-03 6:49:27 - PHP Notice: Undefined variable: setting in /var/www/vhosts/[our domain].hk/PE/vqmod/vqcache/vq2-admin_view_template_common_header.tpl on line 398

How could this problem be corrected?

we have a cache module installed and I wonder if it could be affecting the file/folder path change? if this is the case, I wouldn't know how to properly deactivate the module before I make .php file content and folder path changes properly.