CP redirecting to invalid login URL after upgrade to 2.8.1

Question

I've just run the installer to upgrade from 2.3.1 to 2.8.1

Now when I try to go to the Control Panel (admin.php), I am redirected to

admin.php?/cp/login?return=

Which gives the "Disallowed Key Characters" error.

Answer 1

Turns out this is a known bug with 2.8.0+, affecting users with uri_protocol configuration setting set to ‘PATH_INFO’.

There is a patch available here:

https://support.ellislab.com/bugs/detail/20192/when-uri_protocol-is-path_info-cant-log-into-control-panel-disallowed-key-c