Apache up in flames (AH00025). How to fix?

I was trying to get Apache 2.4.10 to do basic authentication (under HTTPS) as specified in a .htaccess file. I changed one directory's settings to AllowOverride all, and it started serving up nothing but plain vanilla 500 pages. Reverting that directory to AllowOverride none does not seem to have fixed things.

The errors I have found in the logs all seem to be interpreting the requested relative URL as an authorization username, and croaking:

[Wed Jun 22 20:34:00.565683 2016] [core:crit] [pid 24994] [client ____:51295] AH00025: configuration error: couldn't check user: /writing/icons/rublev_trinity_icon.gif, referer: ____

What does AH00025 mean? I've found a few pages like https://httpd.apache.org/docs/trunk/upgrading.html, but not a clear definition of the error code.

And what /etc/apache2/mods-enabled/auth* files do I want?

Please load the module, it will resolve your issue;

LoadModule authz_core_module modules/mod_authz_core.so