Very curious. The error indicates that the filemtime() could not get a time for the file: /home/domeny/domain.com/web/subdomeny/www/wp-content/themes/xytheme/img/attachment_icons/docx.png
Either the file was removed between the directory catalog and the filemtime()check (unlikely) or possibly there are permissions issues.
What would make this intermittent is that the plugin only checks themes and plugins for changes as sparingly as possible and it checks only when you are in wp-admin (so it doesn't slow down your readers). I would check the permissions on the containing directory and that file and see if the owner and group are ok and the permissions make is readable for the apache user.