Automatic Update: public_html/wp-admin/css/theme-editor.dev.css
If you try to upgrade your WordPress installation with the Automatic Update option and after 2 to 7 minutes you get this error:
Could not copy file.: /public_html/wp-admin/css/theme-editor.dev.css
Installation Failed
Other sites advise to change the file permissions to 777 or 755, or to deactivate all the plugins, or to delete the upgrade directory in the wp-content. None of these solutions worked for me.
Solution: Change the FTP Server to ProFTPD
If you manage your server you have to change your FTP Server in the WHM, or if you are in a shared server you have to ask to your hosting company to change it (you need to be root for this):
Change the FTP Server from Pure-FTPD to ProFTPD
Once I did this the Automatic Update from WordPress run in 15 seconds, very fast with a successful update 
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Post author: Daniel Semper
Yep, after hours of fighting with chmods and other option, changing the ftp server to proFTPD was THE solution for me. Thanks for the hint..
Been beating my head against keyboard for hours messing with permissions, owners, suphp, etc. etc. This was the answer I needed, thank you!!!
You are an angel! Like the others I tried it all. One simple switch of the FTP server and I am now golden.
Thank you!
There’s a patch (http://core.trac.wordpress.org/ticket/10913) for WordPress that should go into 3.2 and help fix this upgrade error for most people running into it. It only seems to happen with servers running Pure-FTPd. In the meantime, there’s a plugin that should help fix updates for most people: http://wordpress.org/extend/plugins/ftp-upgrade-fix/
Thank you for posting this tip – it worked like a charm for me!
Works instantly!
Thanks hugely heaps lots for this info. The fix took only a few seconds in WHM!
Very grateful to you, thanks.