What version of eShop are you using? The latest version of eSE shows an echo statement on line 370 which is only triggered if the store is in Test mode.
Line 961 has a
setcookie() call which should be called before anything has been printed. From what I can tell, this only happens if a user-defined
eshop_use_cookie filter has been added and returns true.
Although the error message points to
override_eshop_order_handle.php which is indeed part of the eSE suite, that portion has not been changed – the issue should persist even if you uninstall eSE.
Please try updating your version of eShop if it’s not already the latest. Otherwise, check that you are not unnecessarily using cookies having added the filter yourself. If you can’t get away with NOT using cookies, please check with Rich over at quirm.net, as what you see on line 370 of my file is the same as line 1139 of