After commit 4dcc8cabae670e0ee57a489e7e13c2f93d99753a, the resource locator does not recognize admin template files from extensions. For example, if my extension has a acp module (test_module.php) that uses template file acp_test.html located in root/ext/my/extension/adm/style/acp_test.html, visiting this page in acp results in an error (resource not found in...)
Globalizing the admin path doesn't seem to work well with locating admin template files from extensions. Adding $phpbb_admin_path = (defined('PHPBB_ADMIN_PATH')) ? PHPBB_ADMIN_PATH : './'; back in adm/index.php seems to fix this.