SL access rights can be defined at both the group and user level. The user level trumps the group level. I am thinking that someplace your rights to that screen are limited. I suggest you run the Access Rights report and apply a select filer on vs_accessrights.screennumber equal 0127000. This will show each user and group and what rights they have to that particular screen. since user rights trump group rights, see if you are listed on that report. If so, does it show that you have update and insert rights. If not, you will need to add that at your user id level. If you do not show up, look at the various groups that have rights and see if any of them do not have update and insert rights. If there are such groups, use the user screen specifying your user id and see if you are a member of any of those groups. If you are a member of more than one group and one of those groups only has view rights, this could be your problem. I am not 100% sure how SL handles users who are members of more than one group but I suspect precedence is determined alphabetically on the group id but it is also possible that it is determine based on the most restrictive group.
The bottom line is that it sure sounds like access rights is restricting your capabilities on that screen.