EasyIndex can use a significant amount of server (PHP) memory when it generates thumbnails. Before it starts the thumbnail generation process, EasyIndex attempts to set a server memory limit that will allow it to run. By default it sets a limit of 256 Mbytes and this should be plenty, but some themes and/or plugins use an unusually large amount of memory and there's not enough left for EasyIndex.
If you get "out of memory" errors or you have an index page that is completely blank, try upping the "Thumb memory" setting on the General Settings tab of the EasyIndex settings page to 384 or even 512.
If increasing the Thumb memory doesn't help, it may be that you have one or more posts that have huge images.
The larger the source image is, the more memory the graphics code will need to create a thumbnail from it. With a server PHP memory limit of 256M, EasyIndex should be able to handle most images but if you have really large images (say more than 2,000 pixels high and/or wide) then EasyIndex might still run into memory problems. The best solution in this case is to reduce size of the source image(s). Large image sizes are a Very Bad Thing generally - they add significantly to your page load times and that will adversely affect your search engine rankings.
If you still have problems
If you still have problems even with reasonable image sizes and (say) 512 Mbytes of Thumb memory configured, then you can try increasing the memory limit further although there'll be an upper limit that you can set, depending on your server. You might also try de-activating some other plugins temporarily while you run the generator (the memory limit has to accommodate all the core WordPress code, plus the theme code, plus code from all the active plugins). Another solution is to explicitly set a much smaller index image for posts with large images (there's an "index image select" panel on the post edit page)