All Changes Fail — Without Errors

When I go to update the plug-ins on my site the update-core.php?action=do-plugin-upgrade gives me nothing but “updated successfully” messages. However, when I return to the plug-ins page, nothing has actually updated.

It’s the same if I try to delete a plug-in or a theme: wordpress tells me it has successfully happened, but nothing actually changes.

Other than that the blog works fine with making new pages and comments.

Any help much appreciated.