MacBook doesn't sleep since Mountain Lion

Question

Since I installed Mountain Lion on my MacBook (alu, late '08), it doesn't sleep anymore. When I enter sleep mode, it looks like it's entering sleep mode, but it doesn't fall asleep properly. The screen turns off, but the fans keep blowing and it won't go "off".
Have tried SMC & PRAM rest, no luck. Also tried deleting some dock files (read that in another discussion), no luck either.
Any ideas?

Answer 1

Got the following messages:
20-08-12 20:27:03,975 WindowServer[80]: CGXSetWindowBackgroundBlurRadius: Invalid window 0xffffffff
20-08-12 20:27:03,976 loginwindow[47]: find_shared_window: WID -1
20-08-12 20:27:03,977 loginwindow[47]: CGSGetWindowTags: Invalid window 0xffffffff
20-08-12 20:27:03,977 loginwindow[47]: find_shared_window: WID -1
20-08-12 20:27:03,977 loginwindow[47]: CGSSetWindowTags: Invalid window 0xffffffff
20-08-12 20:27:03,988 loginwindow[47]: ERROR | -[LWBuiltInScreenLockAuthLion preLoad] | guestEnabled is true, but guest mode is not set to a usable value
20-08-12 20:27:14,194 WindowServer[80]: Created shield window 0x25f for display 0x04272240
20-08-12 20:27:14,194 WindowServer[80]: device_generate_desktop_screenshot: authw 0x7fcfb967bfb0(2000), shield 0x7fcfb9673b80(2001)
20-08-12 20:27:14,264 WindowServer[80]: device_generate_lock_screen_screenshot: authw 0x7fcfb967bfb0(2000), shield 0x7fcfb9673b80(2001)
20-08-12 20:27:43,034 loginwindow[47]: ERROR | -[LWScreenLock(Private) screenIsLockedTimeExpired:] | No lock state found, use built in check
20-08-12 20:27:43,273 WindowServer[80]: handle_will_sleep_auth_and_shield_windows: no lock state data