If you've set up a custom thank you page, it might sometimes show you a 403 Forbidden Error or a 404 Not Found error even when the page actually exists.
You may also notice that your Thank You page has some extra data in the URL such as below:
The reason you see this error message is because Imnica Mail passes data to your thank you page that you can use to customize your Thank You page, such as displaying their name on the page. However, on some hosts, it isn't supported.
All it takes is a request to them to whitelist 'mod sec' on the domain in question and the problem is solved.
Some hosting companies will do it directly for you, and others will ask you to add a line to .htaccess in your root folder of your domain. Each hosting company is different, and you will need to check with yours on how to do it.
In case your thank you page redirects perfectly, but you don't want your subscribers to see the extra data in the URL, you can disable the XML data by checking the box "Disable XML In URL" in your list settings.