Shared calendar: one of the users cannot save

[j.c.f.]

Greetings,

Any hints?
1 - Created a shared folder (user.Calendar)
2 - Allowed ACL anyone all
3 - Configured kontact to create the Groupware folder in user.Calendar for 
user A
4 - Did the same with user B
5 - User A creates events and updates calendar normally, user B has access to 
shared calendar but cannot save events and tasks. Every time he tries there's 
a popup box saying so.
Why is that?

TIA,

Joe Karma