TYPO3 Core

felogin is using t3lib_div::_GP(). The problem is that you can't use _DEFAULT_PI_VARS in TypoScript.

Solution:
Change the t3lib_div::_GP() calls to $this->piVars[] and keep the _GP stuff for compatibility

Maybe you want a "logout" link in you menu. You can easily create a page "logout" wich is displayed in you Menu. But you have to add a plugin on this page and the user has to click on logut on this page again.
It would be much better if you can set this "logout" action by default to this page with typoscript.
Something like this

1. page 543 is the logout page wich is displayed in the menu
   [globalVar = TSFE:id=543]
   plugin.tx_felogin_pi1._DEFAULT_PI_VARS.logintype = logout
   [global]

(issue imported from #M14121)