TYPO3 Core

I can't reproduce this. In my installation PATH_tslib contains the correct value in TYPO3 version 3.8.1 and 4.0. could it be that it is changed by an Extension?

Are you using a backend modul? I got feedback regarding the extension mwimagemap and there PATH_tslib didnt work on typo3 4.0. As I sayed, I greped for "'PATH_" and I didnt found anything changing PATH_tslib.

I use this workaround:
if (@is_dir(PATH_site.'typo3/sysext/cms/tslib/')) {
define('PATH_tslib', PATH_site.'typo3/sysext/cms/tslib/');
} elseif (@is_dir(PATH_site.'tslib/')) {
define('PATH_tslib', PATH_site.'tslib/');
} else {

// define path to tslib/ here:
        $configured_tslib_path = '';

// example:
        // $configured_tslib_path = '/var/www/mysite/typo3/sysext/cms/tslib/';

define('PATH_tslib', $configured_tslib_path);
}

if (PATH_tslib=='') {
die('Cannot find tslib/. Please set path by defining $configured_tslib_path in '.basename(PATH_thisScript).'.');
}

the directory tslib contains only FE rendering stuff so it's quite logical that its path is not needed in the BE. If your extension - for what reasons ever - needs the PATH_tslib constant you have to define it in your extension.
But thats not a TYPO3 bug.

ok, I'm sorry, please resolve this ticket

see notes