.htaccess AuthUserFile has no effect or is being ignored

After hours of searching the web and trying dozens of unsuccessful solutions - here is my question.

I'm currently configuring a webserver on RHEL 6.4 and httpd 2.2.15 behind another RHEL 6.4 server using squid 3.1.10 and HTTPS only. I'm also using mod_rpaf to simplify logging and identification of visitors behind the proxy.

My problem is to configure a simple password protected folder. When I try to access the folder, the password dialog pops up with the configured AuthName. So I know that the .htaccess is being parsed. But the dialog does not accept the correct credentials and gives me an error 401.

I messed around with:

different permissions for .htaccess, .htpasswd and parent folders
different absolute locations for the .htpasswd
all activated Apache modules that are available on my system
different encryption algorithms for .htpasswd (crypt, md5, sha, salted sha...)
AllowOverride All on the protected and parent folder

But what I really do not understand that even if I put a wrong location for AuthUserFile there is no error message in Apaches error_log like the well known Permission denied: Could not open password file. Even on LogLevel debug Therefore I think that something is wrong with that Directive AuthUserFile.

I hope there is someone out there knowing better methods to identify the problem.

This is my simple .htaccess I'm using for testing:

AuthType Basic
AuthName "Test123"
#AuthUserFile /var/www/test/.htpasswd
AuthUserFile /notexisting
Require valid-user

Solution

Finally I got it to work!
I tracked the error down to the squid reverse proxy by using lynx on my webserver and successfully accessing the protected folder from there.
With my new focus on squid I started googling again. Already the first link took me to the correct answer: squid did not allow the apache to handle user authentication.

Resulution:
Add login=PASS to the cache_peer command in your squid.conf