I'm having the same problem: I've tested it on versions 1.7.8, 1.7.6, and 8.2, on different servers, and the problem persists. It seems to be a long-standing issue, but not many people use that option. Analyzing the code, there seems to be a problem with the MailThemeController.php controller location in /src/PrestaShopBundle/Controller/Admin/Improve/Design, but I haven't been able to find the problem.
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I'm having the same problem: I've tested it on versions 1.7.8, 1.7.6, and 8.2, on different servers, and the problem persists. It seems to be a long-standing issue, but not many people use that option. Analyzing the code, there seems to be a problem with the Mail Theme Controller.php controller, but I haven't been able to find the problem.