I was going insane also.
What I did next was install a fresh copy of 1.5.3 in a sub-folder and then upload zc_install from 1.5.4 to the sub folder and try the upgrade. This way you find out is it your shop or something wrong with your server setup.
If still bad try http://www.zen-cart.com/showthread.p...34#post1270834 . I did a fresh install of 1.5.3 in a sub folder and another fresh install of 1.5.4 in another sub-folder and then just changed the 1.5.4 configuration files to point to the 1.5.3 database if you don't want to use your real store. Grabbed the hashes in phpmyadmin and run upgrade to get hashes again.
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