File permissions…

Help!!!
Since I mirgated to version 2.6.1 (DE), i have real trouble with my pages. In fact, I get a 403 code (Access forbidden) when I try inserting an image to my pages or if I want to re-edit existing pages.

I suppose I didnt give the right permissions to one or more of the files in wordpress…

Can anyone give me a clue what files that could be…????

PS: In general, WordPress became ulta-slow since I migrated to the new version… is that anyhow connected with this problem?

TX alot !!!