Magento is a popular e-commerce platform that allows businesses to create and manage online stores with extensive functionality and customization options. Server migration is a common step for many e-commerce stores seeking better performance, security, or simply transitioning to a more advantageous hosting solution. However, this process can sometimes bring unexpected complications, such as an error when trying to log into the Magento admin panel.

Causes and Solutions

The most common causes of this issue usually relate to incorrect server configuration, changes in paths, or problems with sessions and cookies. After server migration, it's important to check and possibly update Magento's configuration files, such as env.php and config.php, which contain important information about the database and file paths.

Change in URL or path can also cause login issues. Magento stores basic URL information in the database, and if the domain or path to the Magento installation changes after migration, it's necessary to update these values directly in the database or via command line.

Issues with sessions and cookies are another common reason why you may be unable to log into the Magento admin panel. Check the PHP settings related to sessions and cookies, especially the session.save_path and cookie_domain parameters, which must match the new server environment.

Fixing the Error

1. Database Update: Use phpMyAdmin or another database tool to update values in the core_config_data tables related to basic URLs and paths.

2. Clear Cache: Magento heavily utilizes caching to improve performance. After configuration changes, it's essential to clear the cache to reflect the changes. This can be done either via command line or manually by deleting the contents of the var/cache directory.

3. Server Configuration: Ensure that the new server is properly configured to run Magento, including PHP, database, and web server settings. Some specific Magento requirements for server configuration may be overlooked during migration.

Magento server migration is a complex process that requires careful preparation and inspection. Login errors in the admin panel are a common issue, but they can be addressed through proper diagnosis and configuration adjustments. It's important to stay calm and systematically go through potential causes and their solutions. If issues persist, don't hesitate to reach out to Magento technical support or a professional Magento developer who can assist in completing the migration without further complications.