500 internal server error after upgrade to 3.5.1

Hi. I upgraded, and since then I get 500 internal server error everytime I log into dashboard, although the website loads fine (front page).
I renamed plugins folder, and then when going to dashboard I get an error that I need to “reinstall w3-total-cache or remove advanced-cache.php from wp-content.
If I do that, or if I just put w3-total-cache plugin folder inside “Plugins” directory, I still get the 500 error.

What’s wrong ?