Cant load pluggins, please help!

I upgraded my wordpress to 2.6.2, but now i get an error when i try to go to some pluggin page. I get “Cannot load plugin x”. Examples:

Cannot load nggallery.

Cannot load wpstats.

I tried to go back to 2.6, but the problem persists. I tried to work with the less complicated pluggin, so i get to wpstats and reinstalled it. It asks for an API key, but when i click on the link to insert it i get:

Cannot load wpstats.

I put the API key manually on the archive, and now it dont asks me about that, but there is no WPstats on meu admin page (i mean the link with the stats of my blog).

How can we solve this? Please help…