Checkout page not found

-- url of the page with the problem -- : Under Construction
-- HikaShop version -- : 2.3.0
-- Joomla version -- : 2.5.19
-- PHP version -- : 5.3.28
-- Browser(s) name and version -- : XXXXX x.x.x
-- Error-message(debug-mod must be tuned on) -- : Error_message

Hi,
When I add a product to the cart a pop up (?popup=1) displaying "continue shopping" and "proceed to check out"
After few seconds it disappears automatically. If I manage to click on "Proceed" I get a message :
Page not available (url ......./checkout) at Chrome
and at FireFox : Content Encoding Error - The page you are trying to view cannot be shown because it uses an invalid or unsupported form of compression.

I have seen it working properly, but I must have done something wrong that I can not figure it out
any advise please?

I have done several checks but it did not note help.
First issue is that the pop up after few seconds disappears automatically and
second is that page checkout gives an error of "Page not available"
Can you please tell me which was the default setting ?
or if you want backend access (you already have one) I can remind you the credentials

Hello,
1.
- Did you tried to edit the "force a menu on checkout" option through "Hikashop->System->Configuration->Checkout" like Nicolas told you ?
- Are you using any SEF module ? if yes can you disable it and try it again ?
If it still doesn't work, can you give me a temporary access to your back through a private message so that I can directly test it ?
2.
Note that you'll be able to control your popup display time by using the "Notice popup display time" option through "Hikashop->System->Configuration->Main"

1. when "After a product is added to the cart" is set to
“Always go to check out” : directs to …../index.php/hikashop-menu-for-user-control-panel/checkout and works fine
“Always stay to the same window” stays to the window and the HikaShop Cart Module as well works fine
“Stay and display a popup notice” then the button "proceed to check out" directs to same url as above but I get a Page not found. Please note that HikaShop Cart Module also directs to same url and I get a Page not found.

- I do not use any SEF module

2. As there is no On click option, I have set it to a very long display time and works fine

So I send you credentials if you are so kind to check it out for me

Hello,
I just tested it through your website and it seems like your configuration is OK. At this moment I will recommend you to check the PHP error log for the PHP error message resulting in that 500 error and that will give information on what happened.
If you don't know where that file is located, you'll have to ask your hosting company.