Messed up backend (Magento)

Error

The Smart Slider's pages are messed up, or blank.

1
Cache folder isn't writeable

This error can happen, when our cache folder isn't writable. You should go to the /media/nextend folder, and change the folder's permissions to 777. You can do this for example with Filezilla, connect to your ftp, look for this folder, right click on it and click on File permissions…

Change the Numeric value to 777, and check the Recurse into subdirectories (Apply to all files and directories).

If setting the cache folder to 777 does not help either, check out this documentation, that might help.
2
PHP error or a server limitation
Use Chrome or Firefox, and in your backend press F12, click on Console, and refresh the page. It will write out 404 or 403 errors, either pointing to wrong locations of the files, or telling, that the files are not accessable by php. Contact your server host, and ask their help with it. Or in Magento the wrong .htaccess file usage is a common issue too, so if you have customized .htaccess files, remove them from your server to see, if the problem happens because of them.