Sometimes we try to field one that just requires hunches and long shots that might trigger an idea that solves it, and you seem to have one of those. A while ago JNeuhoff saw it, a couple of times, so he'll be thinking about it. Whatever the mismatch is it is subtle, among OC in a stable installation, tool in a dead match, and server with support paying well informed attention.
The tool for it runs well on 1.5.5.1 when that is running smoothly, and the only other approach I can think of at the moment to OC itself is to review 1.5.5.1 CE RC1 (RC2 will be out shortly) and decide whether you want to use CE RC. You would pro forma back up database and fileset first, but may need just to upload the fileset, since the install/upgrade script would ordinarily not be necessary to run unless the standard script had stumbled. If just the fileset of CE RC works, then "stop" there. If not, then you could let the CE RC upgrade script run through the database, while keeping the config.php pair as-is but giving it new index.php pair and then rerunning vqmod's installer to rewrite the upper lines of the index.php pair. See
http://forum.opencart.com/viewtopic.php ... 82#p444959
and practicalities at
http://forum.opencart.com/viewtopic.php ... 20#p445975
http://forum.opencart.com/viewtopic.php ... 40#p450692
In reviewing the thread, I'm seeing failure to export, as you never got to the point of trying to import, so your Excel version is not yet on the table. That usually IS NOT a problem and would not arise anyway till you reach the open or import step. Toward that eventuality, opening, amending, and then importing, you might well have no difficulty. However, another point to look at if import balks will be your Excel version relative to the .xlsx version (many file extensions do have versions or "flavors" -- .tif/.tiff being among examples of numerous flavors that many graphics programs and file converters cannot open or save). Excel versions cannot all handle all .xls and .xlsx file extensions (in part, .xlsx has embedded .xml elements that aren't in prior .xls). If that problem arises, then if you can, try another Excel version. We're not that far along, yet, since you're still stuck on initial export.