I think you need to put a PW per forum. I never tryed PW of the parent and look to see if the child forums are PWed.

Access Options
Apply This Password to Child Forums
(Only if changed) Yes No

Can Have Password
(Setting this to no will prevent a password from being inherited and from being set by moderators)
(Note: You may still set a password manually)

Forum
- Sub Forum

If "Forum" has a PW and "Apply This Password to Child Forums" is set to "yes" the Sub Forum will carry the same PW. Your cookie will log you into both forums w/ 1 log-in.

If you have set different passwords per forum, you will be asked to use log-in to each forum. I just tested it on VB3.5RC2.

I mean the other way around. I can ONLY set the password on a parent but not on a child ...

Invalid Forum specified. If you followed a valid link, please notify the administrator

vBulletin 3.5 rc3

set on a parent forum, but not work
not even with the default style

ctsolutions

Ill look @ rc3

Updated Hack

Works with 3.5 Gold.

When setting passwords on sub-forums:

Apply This Password to Child Forums [x] yes []no
(Only if changed)