Error.log is a log file that contains all types of messages (including notices that do not prevent running processes) coming from PHP while the plugin runs export or import.
It is located inside ‘/wordpress/wp-content/plugins/all-in-one-wp-migration/storage/’ directory. Errors and messages that the file contains are related to any problems that the plugin might face during the migration – wrongly set files/folders permissions on the server, conflicting plugins and/or themes, conflicts with PHP version installed on the server, etc.
If export or import runs smoothly, the error.log file will not be created inside the ‘storage’ directory and this can be an indication that All-in-One WP Migration did not face any errors. In case you encounter any issue when migrating your website, you can use the error.log information when trying to find the problem or at least to draw a conclusion if the problem is related to the migration plugin.