linux - unexpected behaviour of Ctrl-a x and Ctrl-a X in screen?! regions, locking

According to the screen manual (version 4.0.2.)

C-a x
C-a C-x     (lockscreen)  Lock this terminal.
C-a X       (remove)      Kill the current region.

But what actually happens when I use it (Screen version 4.00.03 (FAU) 23-Oct-06):

C-a X   locks the terminal and asks me for the password. When I enter the correct passwword, I am back in screen but the region is killed (wtf)
C-a x   does nothing apparently

Please note the differences between x (lowercase) and X (uppercase).

Why is there a mixup between the functionalities of C-a X and C-a x? How can I fix this?

I am on CentOS release 5 (Final).