The ApacheErrorDocument directive should be set correctly for HTTP 401 errors.
(1) message/document
(1) Apache configuration file: 'ErrorDocument 401' directive
L1 11. Web Server Software Obfuscation General Directives p17
CIS Apache Benchmark for Unix v1.7 For Apache Versions 1.3 and 2.0
CIS
2007-07-01
Version 1.7
Prose (PDF)