In "Status report" it just outputs the string "version", which causes problems when querying Drupal version When I go to "Status report" menu, only the string "version" is getting printed, which can cause problems with some modules like Webform.
I have Drupal 6.22, but in the "Status report", Webform writes "Webform requires at least version 6.16 of Drupal core.", which means the problem above can be a possible reason.
How can I fix that?

Here are some screenshots:







EDIT 

The file system.module contains

define('VERSION', '6.22');


on line 11, which is correct, BUT the return value of this function was bool(false) (after checking it with a var_dump()), which means a failure (like when this was defined earlier or sg similar). After that, I put the following line BEFORE the above mentioned define function:

var_dump(defined('VERSION'));die();


and it gave back bool(true), which means this constant has already been defined!
I still don't know where, why and how...

There's a function with the name _drupal_bootstrap_full() (in /includes/common.inc, line 2673), in which module_load_all() function is called. The "VERSION" constant is defined after calling this function (and its value is "version" then), but knowing that doesn't make an advance.
That is weird. What does line 11 of the file modules/system/system.module read? For me, it is define('VERSION', '6.22'). How did you obtain the Drupal source files? Via a zipfile or through a git check out perhaps?