Error 404: when clicking on "add to cart" button

Ok, so it's not a 404 error but an internal server error. It's quite different
In that case, you should look at the logs of your server (as the message says in fact) to see what kind of error it is. The location of the error logs as well as the activation of them is different from one hosting company to the other so you might need to ask them how to get the logs of the problem.
Also, could you try to narrow down the causes of the problem ? Does it happen with any product or only for one product ? Does it happen only while logged in ? Only while logged out ? Both ? If you refresh the page, do you still have the problem ?

As I said, you should get the error logs of the server. It's hard to say anything without the real error. Internal server error just means that apache detected an error and stopped the process. It's a standard error message for pretty much anything. But you need to look at the logs to know what the error was.
It could also be a resources limitation on your server which stops the page when it takes too much time to process it... You might want to try a several steps checkout workflow which will reduce the processing for each step... That might just solve the problem...

Here are some clues about the problem: www.warriorforum.com/main-internet-marke...lp-joomla-issue.html

As I said previously, the problem might come from your hosting shutting the page because it's too resource intensive. Try to change your workflow to a multi step workflow... That might just do the trick...

The checkout workflow option is explained on that page: www.hikashop.com/support/documentation/5...config.html#checkout

Typical

The error is always in the dev's app...

If the guy had read the error lines, he would have seen that these error messages are generated because the 500.shtml file does not exists. That's a file that the server tries to display after an error 500 appears but it is NOT linked to the 500 error itself.
So that doesn't tell us anything!

More information on that in these pages:
drupal.org/node/1077782
support.open-realty.org/showthread.php?2...-500.shtml-not-found

They also suggest to ask more information to the hosting company when such errors occur.

One thing you could try is to setup a localhost web server using wamp server or something similar and make a copy of you website to that localhost web server. If you get the same error, it will be way easier to debug the problem as you should have proper logs of the issue (maybe an incompatibility with something else installed on your website ?). If you don't get the problem in localhost, then, that means that the problem lies in your hosting company's server configuration and we can't do nothing about it.

I can see now that you don't get the error on the checkout page but only if you go to the first step. So that narrows down the problem. What is your checkout workflow now ?

Did you edit any of the views of the checkout?

Could you try the checkout workflow: login_address,shipping_payment,confirm_cart_coupon_status_fields,end
If the error appears on the first step again, then that will mean that there is something with the address view which the server doesn't like.

We'll see. Random errors like this often happen because the page is too resources intensive and that your hosting company has a watchdog in place to kill automatically pages which consume too much resources, or it could also be because there are too much websites consuming resources on your server or both... For example, there is a thread about these problems with goDaddy on drupla.org:
drupal.org/node/569402
Some users note that asking the hosting company to move the website to another server solved the problem... So the problem really wasn't drupal but the server...

Maybe there is less processing done on these websites ? Do you also use hikashop there ?