So in the hopes that another Developer stumbles upon the same phenomenon as me here a quick tip!
Woltlab is not setting the Cookie Domain if it ends with .lan or .local but since i adopted .local as a Development TDL (since .dev is taken by Google) this was rather unfortunate.
If you look at the
setCookie Function in the lib\util\HeaderUtil.class.php file
you can just force the
$addDomain Value to true so the Cookie Domain is set.
I hope i saved someone some time, if so feel free to leave a comment :)