Running as service, continuously giving "No source loaders matched! This shouldn't happen"

I am running logstash as a service, and it seems that an erroneous job is stuck in a queue somewhere, though I cannot find it. The error in the subject appears over and over in logstash-plain.log, no matter what files I have in /etc/logstash/conf.d. Googling for the error message turns up only one hit: the source file in github. So what causes this error? Another error I see repeating in the log is: "An exception happened when converging configuration... InvalidSourceLoaderSettingsError... Can't find an appropriate config loader with current settings."

I noticed another error in the log about access denied, which led me to realize the cause of these unhelpful messages is file permissions on the .conf files. It appears that the correct ownership for the files is root:logstash, so must have logstash group membership.