Can’t add media to my posts

I’m a bit baffled by this one. I already made a couple of posts on my blog in witch I added images without a problem. But since a couple of days, whenever I try to add a picture in the post interface I get the message saying that it’s not possible and that I have to make sure that the parent folder is writable. It should be since I didn’t change anything what permissions is concerned to said folders.

The uploads folder is 755 as I think it should be.
Can anyone confirm this? And if anyone knows how I can solve this please let me know =]

Thank you