After installing the new version I got the error message:
Warning: file_exists(): open_basedir restriction in effect. File(...wp/plugins/wp-plugin-tester/tests/test-email-encoder-bundle.php) is not within the allowed path(s) ...
The cause is in email-encoder-bundle.php, line 47:
if (file_exists($_SERVER['DOCUMENT_ROOT'] . 'wp/plugins/wp-plugin-tester/tests/test-email-encoder-bundle.php'))
Removing the "wp" before "/plugins" solves the problem.