The issue is probably in the savePayment method in the Mage_Checkout_OnepageController class where the HTML output for the review step is added to the JSON.

As you've stated the log shows a JSON error so I did a quick search on that found this thread on SO that might explain the issue if you indeed are running an older version of PHP. Also your screenshots of the JSON output show an empty 'html' key in the JSON so that confirms something goes wrong with adding the html to the output.

Since you're building the shop in the Polish language it might be a weird character that is tripping up the HTML output. To check if this is correct a suggestion might be to revert back to the base english language and see if the error still occurs. If not then the issue might be in one of your translation files.

The issue is probably in the savePayment method in the Mage_Checkout_OnepageController class where the HTML output for the review step is added to the JSON.

As you've stated the log shows a JSON error so I did a quick search on that found this thread on SO that might explain the issue if you indeed are running an older version of PHP. Also your screenshots of the JSON output show an empty 'html' key in the JSON so that confirms something goes wrong with adding the html to the output.

Since you're building the shop in the Polish language it might be a weird character that is tripping up the HTML output. To check if this is correct a suggestion might be to revert back to the base english language and see if the error still occurs. If not then the issue might be in one of your translation files.