Fatal error: Cannot redeclare update_page_cache() (previously declared in /data/

Try:

-deactivating ALL plugins temporarily to narrow down the problem. If the problem goes away, re-activate them individually (one-by-one) to find the problematic plugin(s).
If you can’t get into your admin dashboard, try deactivating plugins manually via FTP or phpMyAdmin.

-switching to the unedited default theme (twenty eleven) for a moment by renaming your current theme’s folder in wp-content/themes. The idea is to force WordPress to fall back to the default theme to rule out any theme-specific issue.
If you don’t have access to your admin area, then access your server via FTP or SFTP or whatever file management application your host provides.

-If the above troubleshooting steps do not resolve the issue, re-upload all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory from a fresh download of WordPress. This should replace your core files without changing your content and settings in wp-config.php file and the /wp-content/ directory.

I have same issue here. and t-p, I already try your suggestion and faced these problems.

First, via phpMyAdmin, I can’t found the line active_plugin in wp_option table which is written in that Instruction you’d linked. So I can’t deactivated plugin via phpMyAdmin

Second, deactivated plugin via FTP, I also try to follow the instruction in that link. But nothing changed too.

Finnaly, I upload fresh download excpet wp-config.php and wp-content and nothing change. But thanks anyway.

I’m having same problem. I turned off all plugins before upgrade and still no go. I can downgrade back to 3.3.2 just fine though.

Boy, I’ve upgraded many times and 3.4.1 is the first causing headaches.

I wonder if it is W3 Total Cache causing probs?

helpful reading: Troubleshooting WordPress 3.4 – Master List

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