Apache configuration error: couldn’t check user. No user file?

I was trying to configure Digest authentication in Apache and I was getting Internal Server Error 500 when trying to access my site. The Apache error log reported: “configuration error: couldn’t check user. No user file?”

Turned out the problem was that I hadn’t installed the Digest auth module. So the fix was:

 # a2enmod auth_digest
 # apache2ctl graceful