For some reason the backup-lucene-index folder is owned by root instead of alfresco (which tomcat runs as). I'm sure this is the problem. Also, the directory that the wiki page was trying to save to is also owned by root – even though all other content store directories are owned by alfresco, including todays.
I have never run this application as root. Any ideas how that could happen? Is it safe to just fix the permissions manually?