404 error when trying to view image attachment pages

Hi.

For a few days now, I have been experiencing problems with our blog.

I can see my uploaded images, as previously, through the media library. I can access each image by clicking on “edit”. But if I then click on “View Attachment Page”, I am taken to the “404! We couldn’t find the page!” message. This applies to all images in the library.

At the same, images in some posts (but not all) are behaving differently from before. It used to be possible for readers to click them and see them full size, whereas now that click simply relads the posts in question.

I have experimented with deactivating plugins, but have not found a solution for this. Does anyone know what might cause this, and what might have triggered it?