Problem Installing Drupal 7.22 - Cannot redeclare render() in [...]/includes/common.inc on line 5978

Dear Drupal-Pros,

I tried to run the installation of Drupal 7.22 but I were not able to. I'm used to do this and I succeeded many times before. So here is what I'd like to do:

I'm developing right now a public website on my own developer server. Later, when I'm happy with the result, I plan to export the developed website from my development server to the public server. I did this before and it never was a problem.

This time I tried, as many times before, to do a clean install of Drupal 7.x on the target server. But I can't access the install.php or index.php. I installed Drupal 6.28 on the same server successfully. I even tried to then upgrade from 6.28 to 7.22 by following the instructions in UPGRADE.txt. But in Step 14 (Running update.php) I were not able to open update.php. The only thing I get is Error 500.

So I decided to log php errors and got the following one:

[20-Apr-2013 15:09:39] PHP Fatal error: Cannot redeclare render() in /home/im/www/public_html_staging/drupal/includes/common.inc on line 5978

Could someone please explain me how to handle my problem. I would like to install Drupal 7.x.

Any help is highly appreciated.

Cheers,

Huppys