gajim do not take in account password send in invitation
Point 7.5 of xep 0045 ( http://www.xmpp.org/extensions/xep-0045.html#invite ) say a service should send the password with the invitation when a user invite another one in a password protected room.
However, gajim do not use this information ( ie, doesn't autofill the dialog with password ), so the user cannot enter the room.
Gajim should detect there is a password and add it directly in the dialog box.
( beware, ejabberd do not send the password, mu_conf does )