You will usually see a 403 error when you are trying to do something that you don't have the correct permissions to do. For example, a 403 error would show if an outgoing committee member tried to complete a section that was for the incoming committee to do.
Incorrect 403 error codes are usually resolved by switching roles - you can do this by using the drop down menu at the top right corner of the page. You will need to select the role relevant to the task you wish to complete. This will usually be switching to the current years role (for those remaining on a committee) or switching to the correct Club/Soc/Project (for those on more than one committee).