Did you check /var/log/lighttpd/error.log In there you might find the reason. In my case it was a config file in /etc/lighttpd/conf.d/ lacking the number prefix a simple renaming of the file solved the problem. This change for lighttpd was mentioned in TOS 6.0 release notes.
In another case (Webapps icons missing) there was no error to be found in the error.log but there were double config files with different number prefixes found in /etc/lighttpd/conf.d/ which caused the problem.