Today I noticed the following lines in the Apache error_log file:
[notice] cannot use a full URL in a 401 ErrorDocument directive --- ignoring!
Of course, we do not like to waste the error_log file so I tried to find a solution to the problem.
In the present case, the problem was caused by the ErrorDocument file that was provided in the document_root directory for the domain name, as follows:
ErrorDocument 401 http://domain.tld/errorpage.html
This row had to be modified as follows: