This may be a mod_security issue. Please check if your host is running it.
You can turn off mod_security by adding below code to .htaccess :
First thing is checking your config.php and database.php. They can be found in /system/expressionengine/config. Check your database credentials.
In the config.php file check the file paths and turn error reporting on.
Also, there might be an alternative config solution such as config bootstrap or master config. If that's the case, update those files with ...
An EE configuration needs to know both the URL of the site and the local path to the webroot folder within the server you are using to operate.
If you have changed servers then it is likely that the local path to the webroot folder will have changed, and as a result EE operations that rely on accessing files stored below that webroot folder (such as images) ...
The web server just needs to pass environmental variables on to PHP. For segment support, this means that PHP needs the ability to access the portion of the URI after the script name, e.g. the foo/bar bit of index.php/foo/bar. Typically this is passed to PHP by Apache as PATH_INFO or ORIG_PATH_INFO.
Try creating new file on the server named pathtest.php ...
I ran into the same error. It was caused by having Imagick upload actions set for some Channel Images fields but not having ImageMagick itself installed on the new server. Once I installed ImageMagick, images uploaded without a problem.
I've run in to this issue before. At your new host, you'll have tempdomain.hostname.com - this can be the primary site. For the secondary site, you may need to grab any old domain (we've used yoursiteinprogress.net) and make that the secondary domain. Update your EE config to point to these domains, and make sure everything works. Then when you're ready to ...
Sounds like your actual upload directory isn't writable. You can check where this is located in the file structure on the File Upload Preferences page of the CP. Permissions should be set to 777. "_assets" probably isn't the actual upload directory but is inside it.
Wanted to update the original post here with the fix.
I had to manually copy over my original cache/assets directory from the original server to the new one to solve the problem. As soon as that was in place all thumbnails reappeared in EE. I hadn't expected that anything in the cache directory should be moved between servers as it may cause conflicts.
If you go to (domain_name)/index.php does the page load? If this works I would suggest looking at your .htaccess file (supposing your server runs Apache), making sure it was copied correctly and nothing needs to be changed.
UPDATE: Since you say your page does finally load but after a crazy number of seconds, I recommend you try the following steps to ...