I’ve replicated this issue in my own install. When operating on locale/sr_SR/default.xml with the “section.default.policy” key, the preg_match() fails for a reason I have not identified.
However, @vvucic, note that the entry in the distributed file ojs/default.xml at ojs-dev-2_4 · pkp/ojs · GitHub does have the closing message tag. In my experience, “missing” the end tag is often a matter of using an editor or terminal that is not Unicode aware. (It really is there, but the display is mangled.) Double check your editor and terminal and confirm against the GitHub source if end tags are “missing”.