Thank you for the post.
I encountered this issue personally and while I am unsure what caused the problem, the resolution was to first repair disk permissions and then to uninstall and reinstall with the full FileMaker Server 13.0v2 installer: fms_220.127.116.115.dmg.
First backup any databases and save your schedules and groups.
Please let me know if any of the above requires additional clarification.
Success! I ran disk utility / repair disk permissions and I got a long list of items needing to be repaired. I then shut down FM server, restarted my computer, tried it again and it worked without having to reinstall. One thing odd was that I had to change the backup folder location not only on the folders screen; I had to edit the backup schedule to point to the new folder.