How to allow a user 'edit' on activities?
I fixed this problem by changing ACLs.  Originally the user was in role 'A' and the contact where they could create but not edit an activity was in group 'B' - and role 'A' had edit access to group 'B'. By changing the target contact to be in the same group as the 'A' role and giving that role 'edit' access to itself (its group) then they get edit access to activities.  As it happened, putting the target contact into this group is OK but this might not always be the case.  Seems like 'edit activities' should be a permission - this has been discussed before.