Forbidden error when saving Checkout in config.

-- url of the page with the problem -- : mybabylonia.gr
-- HikaShop version -- : 1.5.8
-- Joomla version -- : 2.5.4

Hi,
I'm getting a forbidden error when trying to save the checkout in configuration.
What I'm trying to do is add the coupon field in the checkout. Is it possible to do it manually?
Been reading around for this and tried many options found in forum etc with no luck.
I can save joomla's global configuration.

I do believe I found a way to edit and save the checkout process in the past, but cannot remember what I did to fix it.

Any help would be appreciated.

Error:
Forbidden
You don't have permission to access /administrator/index.php on this server.
Apache Server at mybabylonia.gr Port 443

I tried fixing using a php.in file with no luck.

file_uploads = On
memory_limit = 256M
upload_max_filesize = 32M
post_max_size = 32M
max_input_time = 180
memory_limit = 128M
max_execution_time=300;
max_input_vars=10000

Hi,
I rang the hosting support and they changed the post size to its maximum with no luck.
Look at the attached screenshot.

could I add the coupon field by editing code instead of going through the admin area?
thanks

memory_limit 384M
max_execution_time 180
max_input_time 180
post_max_size 128M
upload_max_filesize 128M

Getting the following error in log of Web Application firewall (plesk):

ModSecurity: Access denied with code 403 (phase 2). Match of "eq 0" against "MULTIPART_STRICT_ERROR" required. [line "30"] [id "210240"] [rev "1"] [msg "COMODO WAF: Multipart request body failed strict validation|PE 0|mybabylonia.gr|F"] [severity "CRITICAL"] [hostname "mybabylonia.gr"] [uri "/administrator/index.php"] [unique_id "V8SjIX8AAAEABmaMI8MAAAAx"][file "/etc/httpd/conf/modsecurity.d/rules/comodo/12_HTTP_Protocol.conf"] [line "30"] [id "210240"] [rev "1"] [msg "COMODO WAF: Multipart request body failed strict validation|PE 0|mybabylonia.gr|F"] [severity "CRITICAL"] [hostname "mybabylonia.gr"] [uri "/administrator/index.php"] [unique_id "V8SjIX8AAAEABmaMI8MAAAAx"]

Thanks for that,
I called host support and an exception was added to the firewall and now I am able to save properly.

Thanks again