There is a race condition in the documentation build. You cannot build the english and the german documentation at the same time. Apparently, the share images (ps, eps, png) are not correctly build in an atomic way. Solution: create temporary files during build, and only if it was complete and successfully created, the file will be moved to the final target name.
Created attachment 186377 [details] image build patch
A commit references this bug: Author: wosch Date: Tue Sep 19 09:57:15 UTC 2017 New revision: 50879 URL: https://svnweb.freebsd.org/changeset/doc/50879 Log: fix race conditions in doc image builds PR: 222327 Changes: head/share/mk/doc.images.mk