Strange things writing a post or modifying widgets

Hi everyone,
I installed the latest WP 2.1.2 and several plugins on my wordpress site (SnailGallery, some widgets) and something strange happened. Now I can’t display none of the two HTML editors (I checked user preferences but nothing happens); every time I modify a post a confirmation box takes the whole window (not like the old pop-up style); finally, I can’t modify anymore the sidebar widgets configuration (the widgets simply don’t move!).
I tried disabling all plugins and switching themes (I’m using WordPress 2.1.2 with the Mandingo 1.4.1 theme), but everything stays just the same.
What should I try next?
Thanks in advance