After changing servers, expose which is a joomla gallery component might develop issues with the backend manager login. If you have tried every possible advice you can try this one The cause of the problem might be unwanted spaces that have been introduced to some key files during the file transfer you can check on the following files and remove any blank spaces after the php script closure ?>
1. components\com_expose\expose\manager\amfphp\amf-core\app\Constants.php
2. components\com_expose\expose\manager\amfphp\amf-core\util\AMFBody.php
3. components\com_expose\expose\manager\amfphp\amf-core\app\php5Executive.php
Note that there m ight be other files To know which other files might be involved or if this will be a solution for you use firebug with firefox to capture the login error response on the gateway.php file
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