Every time i edit menu items, my less and css files get destroyed.

I developed many many sites now with T3-Framework, but never had such a strange issue:

On one of my site, every time when i edit a menu item in backend (e.g. modifying title), all my CSS and LESS files get destroyed. They are almost original, as I would have updated the site.

I have no idea, where to start investigating.

Did anyone experience the same issue???