There is still a problem with the function that checks if a class is already loaded into the core. What i - mistakenly - did is autoloading the language library inside the autoloading config file.

The error it produces:

Fatal error: Cannot redeclare class ci_language in C:\server\xampp\htdocs\ci\system\libraries\Language.php on line 27

Off-course this is wrong because the language file is automatically loaded into the core. Maybe an extra check inside the code could be usefull to prevent these slipups. My quess that this is also the problem when autoloading other core classes.