Theme upgrade not showing in Dashboard options

This is an odd thing, because I've uploaded the identical atahualpa351 folder to two different websites with two different results. On one, the upgrade option worked just fine. Both sites are using WP 3.0.

However, on the second website (tanglepatterns.com), when I choose the "Preview" option for 351, the screen is entirely blank. I tried to activate it anyway, and the site went blank too. So I quickly reactivated the 346 version.

I checked the folder permissions and they are identical. I tried disabling one or two plugins, but that didn't fix anything.

So I removed all the 351 files from the server, then uploaded them again thinking maybe something went wrong in the upload. But the problem still persists.

I'm loathe to mess around too much because my site is quite active and I don't want to crash it.

Any idea what the problem might be?

Thanks!