What you see on the Inspector are not Margins but you should see a Width larger than zero. This is the Width of the layout. Margins are specified for a layout in Layout Setup on the Printing tab. You may need to check those settings.
But the error you report has nothing to do with right and left margins, its a function of your currently specified print size, the height of the header and footer parts plus any top or bottom margins specified in Layout Setup on the printing tab.
Try temporarily selecting a different printer and see if anything changes. Check and see if a custom page size is specified and try selecting different page sizes to see if that has any effect on the results that you are getting.
Thanks for the comment Phil.
I have tried selecting different printers and have tried several different page sizes with different printers. Everything seems to want the width of the layout to be "0px".
I'm really getting worried on this one.
Create a brand new file and see if it shows the same behavior.
It may be that your file is damaged. Got a recent back up copy?
I just created a new file and the same problem happens. It sets the width to 0px.
However, my other database is OK. It is just happening to one of my databases and any new database files that I create.
That's very strange. I suggest reporting this over in Report an Issue to see what the FileMaker Techs have to say about this given that you can reproduce this behavior in a brand new file. You might also try re-installing FileMaker to see if that makes any difference here.
Re-Installing FM11 lets me create new files without this problem ... however, my original file still has the problem. My backup was 5 working days ago. So .... I guess I've got some work ahead of me!