I'd wonder if this coud be a solution, because as you said, only XLS docs are affected from downloading out of OWA, but not any other documents. Could you try do deactive (in case it is) the compression for OWA ressources? Afair MSIE (also v7) has the Bug not displaying or fetching content right when using an ssl-connection with compression. But I think there was also something about caching in that contect, but that might be deactivated (or I'd wonder if OWA sends attachments with cache headers). Thus I am not sure if this will help here. You could also find that out, if the error also occurs when using a browser != MSIE, e.g. Firefox 2.
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