Messed up back-end

This problem:

can be caused by various server issues.

Bad htaccess file or security plugin

When you see a messed up page, it can happen that you have a badly configured .htaccess file on your server. The easiest method to figure that out is if you right click on the page → choose Inspect element → go to the Net or Network tab, and refresh the page. You'll see a couple of red lines there:

and beside them, there will be a status code. If it's 404, you should reinstall the plugin, as probably the files are missing. If it's 500 or, as in my case, 403, it is very likely a security plugin or a bad htaccess file. Click on one of the red lines and go to the HTML tab to see more about your issue:

when you see a message like this, that's probably a .htaccess problem, so you should check your folders from left to right to see if any of them has such file. If you find any, rename it and refresh the page. If you found the file that was causing the problem, the page will look fine.

If you would see 500 error it can also happen that some server limitation is reached by something, so you could check your error log (or contact your host for help, because they can resolve the 500 error messages) to learn more on the issue.

HTML or PHP error

In rare cases, it is possible that there is a PHP or HTML error in the theme causing the problem. The easiest method to figure that out is if you right click on the page → choose Inspect element → go to the Net or Network tab, and refresh the page. Probably you won't see red lines on anything special. Click on the URL of your current site and go to the Response tab:

you will see lots of codes, so scroll to the bottom and find something like this:

Open the response in a new tab, right click on it and select View page source. Copy the entire source, find an HTML validator, like https://validator.w3.org/ and paste the copied source, then validate it. Probably you'll see something like this:

which means that somewhere on your admin template, there is an unclosed HTML comment. Locate your admin template, open it with an HTML editor (like Notepad++), find the said line, and search for the HTML comment opening tag: <!--. The first one your fine above your line will cause the problem. You can remove or code it. In my case, I found these codes in my admin template:
<!--[if lt IE 9]><script src="<?php echo JUri::root(true); ?>/media/jui/js/html5.js"></script><![endif]-->
    <!--
</head>

and the <!-- before the </head> was not closed, so I had to remove it to make the slider work again.

In most cases, PHP errors can be checked by turning on the Error reporing on the server. You can do this different ways on on Joomla and on WordPress. After turning on the Error reporting, if your problem originates from a PHP error, it will be written out.

Cache folder is not writable

Possible errors
  • Messed up back-end.
  • Slide editor doesn't work.
  • Live preview doesn't show the slider.
Solution

First check out the framework specific solutions for this problem, because that is the most common:

If you checked out them, and that didn't helped, then this problem usually happens, because of another plugin/extension, which has an error in its code, or it is a security plugin/extension, which comments out javascript codes.

In Joomla there aren't many extensions loading their codes in the back-end of other extensions, probably you have one specially for the admin area. Go to the ExtensionsManage (in older Joomla versions Extension Manager), and unpublish it to see if it solves the problem. If not, then you can check it out with Firebug.

In WordPress there are many plugins loading their codes in other plugins' back-end pages too, so you should try to deactivate them to see, which one causes the problem. If you have a lot of plugins, you can deactivate more together, it might be faster to narrow it down, then to do it one-by-one. Maybe you can try to start with security, image, JavaScript or CSS related plugins.

Messed up backend using AMPPS

First, try to make your site write out the error, which you can do like this on Wordpress, and like this on Joomla. That will give you the idea if you need to turn on some PHP extension, or increase some server setting.

Call to undefined function ctype_digit()

You should enable the ctype extension at AMPPS Application → PHP Tab → PHP Extension. Check ctype and Apply.