With recent version of gallery2 (at least with 2.3) password protected albums do not work anymore. This means that after entering the correct password the user sees a blank page instead of being logged in and forwarded to the gallery.
I found a post in gallery’s forum describing this issue. There seems to be a problem with SSL/Cookie-Handling. The captcha plugin is also concerned.
However the quick fix presented by valiant works for me:
but there's a easy fix:
just add GalleryCoreApi::requireOnce('modules/core/classes/GalleryTranslator.class'); right before line 780 in modules/core/classes/GallerySession.class.
Unfourtunately up to now (2009-05-10) the fix is not yet included in trunk.