On a PrestaShop 18.104.22.168 installation I purchased the One Page Checkout 1.2.2 and installed it. After that I got the following error message when trying to add a product to the basket in the shop front end: TECHNICAL ERROR: unable to add the product.
Details: Error thrown: [object XMLHttpRequest] Text status: parserror
Solution: The reason is the web server is running in SAFE MODE and the user owning all the PrestaShop files is the FTP user I use to upload the shop. When installing the One Page Checkout module it will be owned by the user running the web server process (in my case www-data:www-data). In SAFE MODE a php script is not allowed to execute a script owned by another user. The solution is to set the owner of all the order-files in the PrestaShop directory to my FTP-user (including the files installed by the OPC module). In a shell this is simply done by the chmod command:
cd prestashop-installation-dir chown -R my-user:my-group order* (Replace prestashop-installation-dir with the real path where PrestaShop is installed and my-user:my-group to the userid:grouid owning the rest of the files in the Prestashop installation.)