Hmm...
Just a stab in the dark, but are there any merged cells
that extend below the header section? Maybe something like
that is interfering with the pane definitions.
Try removing all formatting from the top area of the
spreadsheet (select entire rows and use
the "Edit\Clear\Formats" menu command) and see if the
problem persists. If that does not solve it, I would try
copying everything to a new spreadsheet in case there is
some internal corruption in the workbook.
Good luck,
Tom