“null is null or not an object” error message

hello

I have a simple photo gallery with some posts arranged by categories.
Site is using the modularity theme with the built-in menus for easy category access.
When loading the website in IE6 I get the following error:
‘null’ is null or not an object and the menus don’t work.
When using chrome or firefox, however, the problem is gone and the menus work fine.

any ideas? I suspect the problem relates to the header.php file.
below is the file content:

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