Hi Jorge,
that's not a configuration problem, but a problem with PHP 5.5/5.6 and the Guzzle framework, I think.
PHP 5.5
213.73.x.x - - [21/Dec/2015:22:44:49 +0100] "OPTIONS / HTTP/1.1" 401 356
213.73.x.x - - [21/Dec/2015:22:44:49 +0100] "OPTIONS / HTTP/1.1" 401 283
[2015-12-21 22:44:49] {"request":"[object] (GuzzleHttp\\Message\\Request: OPTIONS / HTTP/1.1\r\nHost: caldav.g?h?l.de\r\n
User-Agent: AgenDAV/2.0.0-beta1\r\n\r\n)","response":"[object] (GuzzleHttp\\Message\\
Response: HTTP/1.1 401 Unauthorized\r\nDate: Mon, 21 Dec 2015 21:44:49 GMT\r\n
Server: Apache/2.4.17 (Win32) PHP/5.5.30 OpenSSL/1.0.2d\r\nX-Powered-By: PHP/5.5.30\r\n
WWW-Authenticate: Digest realm=\"SabreDAV\",qop=\"auth\",nonce=\"567????149403\",opaque=\"df????ff8cf60599c939187d0b5c54de\"\r\n
Content-Length: 283\r\nContent-Type: application/xml; charset=utf-8\r\n
Content-Language: de\r\n\r\n
<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<d:error xmlns:d=\"DAV:\" xmlns:s=\"http://sabredav.org/ns\">\n
<s:sabredav-version>3.0.5</s:sabredav-version>\n <s:exception>Sabre\\DAV\\Exception\\NotAuthenticated</s:exception>\n
<s:message>Username or password was incorrect</s:message>\n</d:error>\n)","exception":"[object]
GuzzleHttp\\Exception\\ClientException(code: 401): Client error response [url] http://caldav.g?h?l.de/ [status code] 401 [reason phrase]
Unauthorized at X:\\CalDAV\\AgenDAV2\\web\\vendor\\guzzlehttp\\guzzle\\src\\Exception\\RequestException.php:89)"}
{"url":"/calendar/login","ip":"213.73.x.x","http_method":"POST","server":"www.g?h?l.net","referrer":"https://www.g?h?l.net/calendar/login"} >>>>>>>>
OPTIONS / HTTP/1.1
Host: caldav.g?h?l.de
User-Agent: AgenDAV/2.0.0-beta1
<<<<<<<<
HTTP/1.1 401 Unauthorized
Date: Mon, 21 Dec 2015 21:44:49 GMT
Server: Apache/2.4.17 (Win32) PHP/5.5.30 OpenSSL/1.0.2d
X-Powered-By: PHP/5.5.30
WWW-Authenticate: Digest realm="SabreDAV",qop="auth",nonce="56????d149403",opaque="df????ff8cf60599c939187d0b5c54de"
Content-Length: 283
Content-Type: application/xml; charset=utf-8
Content-Language: de
<?xml version="1.0" encoding="utf-8"?>
<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns">
<s:sabredav-version>3.0.5</s:sabredav-version>
<s:exception>Sabre\DAV\Exception\NotAuthenticated</s:exception>
<s:message>Username or password was incorrect</s:message>
</d:error>
--------
Client error response [url] http://caldav.g?h?l.de/ [status code] 401 [reason phrase] Unauthorized
===================================================================================
PHP 5.4
213.73.x.x - - [22/Dec/2015:13:55:19 +0100] "OPTIONS / HTTP/1.1" 401 356
213.73.x.x - - [22/Dec/2015:13:55:19 +0100] "OPTIONS / HTTP/1.1" 200 -
[2015-12-22 13:55:19] {"request":"[object] (GuzzleHttp\\Message\\Request: OPTIONS / HTTP/1.1\r\nHost: caldav.g?h?l.de\r\n
User-Agent: AgenDAV/2.0.0-beta1\r\n\r\n)","response":"[object] (GuzzleHttp\\Message\\
Response: HTTP/1.1 200 OK\r\nDate: Tue, 22 Dec 2015 12:55:19 GMT\r\n
Server: Apache/2.4.17 (Win32) PHP/5.4.38 OpenSSL/1.0.2d\r\n
X-Powered-By: PHP/5.4.38\r\nX-Sabre-Version: 3.0.5\r\n
Allow: OPTIONS, GET, HEAD, DELETE, PROPFIND, PUT, PROPPATCH, COPY, MOVE, REPORT\r\n
DAV: 1, 3, extended-mkcol, access-control, calendarserver-principal-property-search, calendar-access, calendar-proxy, calendarserver-subscribed, calendar-auto-schedule\r\n
MS-Author-Via: DAV\r\n
Accept-Ranges: bytes\r\nContent-Length: 0\r\nContent-Type: text/html; charset=ISO-8859-1\r\n
Content-Language: de\r\n\r\n)"} {"url":"/calendar/login","ip":"213.73.x.x","http_method":"POST","server":"www.g?h?l.net",
"referrer":"https://www.g?h?l.net/calendar/login"} >>>>>>>>
OPTIONS / HTTP/1.1
Host: caldav.g?h?l.de
User-Agent: AgenDAV/2.0.0-beta1
<<<<<<<<
HTTP/1.1 200 OK
Date: Tue, 22 Dec 2015 12:55:19 GMT
Server: Apache/2.4.17 (Win32) PHP/5.4.38 OpenSSL/1.0.2d
X-Powered-By: PHP/5.4.38
X-Sabre-Version: 3.0.5
Allow: OPTIONS, GET, HEAD, DELETE, PROPFIND, PUT, PROPPATCH, COPY, MOVE, REPORT
DAV: 1, 3, extended-mkcol, access-control, calendarserver-principal-property-search, calendar-access, calendar-proxy, calendarserver-subscribed, calendar-auto-schedule
MS-Author-Via: DAV
Accept-Ranges: bytes
Content-Length: 0
Content-Type: text/html; charset=ISO-8859-1
Content-Language: de
Best wishes,
Thlomas