List of problematic files:

/core.php:69:

require_once( __DIR__ . '/core/constant_inc.php' );

The "/core" folder is located in a different location. Probably can be replaced with: require_api( 'constant_inc.php' ); And will have to move the file: "/core/constant_inc.php" -> "/constant_inc.php". I don't see any other way out, because the $g_core_path variable can't be used in this place; it's loaded in the same file, but lower.

/core/email_api.php:79:

require_once( __DIR__ . '/classes/EmailMessage.class.php' );
require_once( __DIR__ . '/classes/EmailSender.class.php' );

/core/email_api.php:1580:

require_once __DIR__ . '/classes/EmailSenderPhpMailer.class.php';

/classes/EmailSender.class.php:18:

require_once( __DIR__ . '/EmailMessage.class.php' );

/classes/EmailSenderPhpMailer.class.php:23:

require_once( __DIR__ . '/EmailSender.class.php' );

The "/classes" folder is no longer available. These lines can simply be removed, since the class autoloader is used anyway.

/core/logging_api.php:325:

$t_root_path = dirname( __DIR__ ) . DIRECTORY_SEPARATOR;
...
$t_root_path . 'plugin.php'

The "/core" folder is located in a different root folder. We probably need to use the $g_core_path variable here.

/core/commands/IssueViewPageCommand.php:36:

And 12 files next to it. The "/core" folder is not located next to "/api".

$t_api_path = dirname( __DIR__, 2 ) . '/api/soap/';

Replace with:

global $g_absolute_path;
$t_soap_dir = $g_absolute_path . '/api/soap/';  

/admin/check/check_paths_inc.php:73:

MantisBT thinks that the "/mantisbt-config" folder is inside the "/mantisbt" folder.

$t_new_path['real_path'] = realpath( $t_new_path['config_value'] );

Needs to be replaced with:

$t_new_path['real_path'] = realpath( $t_new_path['config_value'] ) . DIRECTORY_SEPARATOR;

Please add any findings to 0021584

@atrol Thanks for pointing out the issue about this problem. I actually searched for it using the keyword "admin/check/index.php,". But that report listed the old URL "/admin/check.php."