michalblazek

WordPress update seems to be basic stuff. You just click Update now and that’s all. Unfortunately we have seen countless WordPress errors after various updates or rutine changes.
These are usually caused by using some deprecated codes or not compatible plugins. Sometimes there may be some issue with a server memory, PHP or database. We have recently started to use the checklist for WordPress core, plugins or themes update. 
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WP Update Checklist

Checklist for WordPress core, plugin updates and themes updates. What should your developer always check.

John Ford

Some server configurations running Pure-FTPd have trouble updating WordPress via FTP. Common error messages when trying to update WordPress or update/install a plugin look like this:
<ul>
<li>Could not copy file.: /wp-admin/css/theme-editor.dev.css</li>
<li>Could not copy file.: /public_html/wp-admin/css/theme-editor.dev.css</li>
<li>Could not copy file.: /…/…/wp-admin/css/theme-editor.dev.css </li>
<li>Could not create file.: /public_html/</li>
</ul>
Each of these cases seem to be related. There is a patch going into WordPress 3.2 that should take care of the issues and the plugin will be obsolete after updating to that version (https://core.trac.wordpress.org/ticket/10913). In the meantime, install and activate this plugin. Updates should work as expected.

FTP Upgrade Fix

Fixes problems that prevent installing or updating WordPress, themes, and plugins on some FTP server configurations (typically Pure-FTPd).

WP Update Checklist vs FTP Upgrade Fix