@amanzoni - Please provide the following information in a bug report to us here, also please include temp. login information to your site & steps to reproduce the error, so our developers can take a closer look at what may be happening.
- WordPress up to date?
- NextGEN up to date?
(please confirm the following, with your server host tech) :
- MySQL up to date (5.2.4)?
- PHP up to date (5.0)?
- The mod_rewrite Apache module activated?
- PHP Safe Mode OFF?
and if you're willing to share:
- Who are you hosted with?
- What theme are you running?
- Which plugins do you have installed and activated?
- Does this error still occur if you switch your theme to the WP default, and deactivate all plugins except NextGEN Gallery? (*be sure to clear your browser cache and reload your site after deactivating)