Permissions issue

I have an strange problem in one of my websites. I have a website installed in a virtual server in virtualmin (wordpress). All directories are in 755 and all files are in 644. The owner of everything is the virtual server admin (it's called "welcome"). The problem is that any file can write anywhere (file upload, file writing, error_log ...) and I don't know what can i do more...

Anyone have an idea ?

Other thing, everything worked well a week ago, I don't know what's changed.

Thanks everybody in advance !!