Working on a Atahualpa site for my brother. http://neologisticsediting.com/

Site has been running fine for years, but lately the blog posts are not rendering. If you click on a post, you get nothing but a blank white page. Other pages and functions seem to be working normally. Any suggestions?

What version of atahualpa?
disable all plugns and see what happens

Wait...are you really using version 3.4.9 ????
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really???
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the current version is 3.7.24!

3.4.9 is correct. I don't know if or when he might have upgraded the site last. Is this version fundamentally incompatible with current WP version?

I did disable all plugins with no improvement.

Atahualpa version 3.4.9 was released on May 12th 2010 that is 6 years ago!

Would you really expect anything that was released 6 years ago to still work with the current version of WordPress?

You WILL need to rebuild your sidebars when you upgrade so make a list of which widgets you have in the sidebar. You may want to do this on a replica of the site first.