Can not use LDAPCredentialsProvider

When I try to use LDAP to authorize users in A3, I get the exception listed below.
Here is what I added to osuser.xml:

<provider class="com.opensymphony.module.user.provider.ldap.LDAPCredentialsProvider">
<property name="java.naming.factory.initial">com.sun.jndi.ldap.LdapCtxFactory</property>
<property name="java.naming.provider.url">ldap://dedus-ad02.valtech-intern.de:389</property>
<property name="searchBase">DC=valtech-intern,DC=de</property>
<property name="uidSearchName">sAMAccountName</property>
<property name="java.naming.security.principal">CN=LDAPReader,CN=Users,DC=valtech-intern,DC=de</property>
<property name="java.naming.security.credentials">*****</property>
<property name="exclusive-access">true</property>
</provider>

05.12.2003 20:54:46 com.opensymphony.user.util.ConfigLoader addProvider
SCHWERWIEGEND: Could not create instance of provider
java.lang.ClassNotFoundException: com.opensymphony.module.user.provider.ldap.LDA
PCredentialsProvider
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoa
der.java:1444)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoa
der.java:1289)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:302)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:141)
at com.opensymphony.user.util.ConfigLoader.addProvider(ConfigLoader.java
:117)
at com.opensymphony.user.util.ConfigLoader.access$100(ConfigLoader.java:
37)
at com.opensymphony.user.util.ConfigLoader$ConfigHandler.endElement(Conf
igLoader.java:154)
at org.apache.xerces.parsers.AbstractSAXParser.endElement(Unknown Source
)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanEndElement(
Unknown Source)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl$FragmentContent
Dispatcher.dispatch(Unknown Source)
at org.apache.xerces.impl.XMLDocumentFragmentScannerImpl.scanDocument(Un
known Source)
at org.apache.xerces.parsers.XML11Configuration.parse(Unknown Source)
at org.apache.xerces.parsers.DTDConfiguration.parse(Unknown Source)
at org.apache.xerces.parsers.XMLParser.parse(Unknown Source)
at org.apache.xerces.parsers.AbstractSAXParser.parse(Unknown Source)
at javax.xml.parsers.SAXParser.parse(Unknown Source)
at javax.xml.parsers.SAXParser.parse(Unknown Source)
at com.opensymphony.user.util.ConfigLoader.load(ConfigLoader.java:59)
at com.opensymphony.user.UserManager.<init>(UserManager.java:107)
at com.opensymphony.user.UserManager.<init>(UserManager.java:58)
at com.opensymphony.user.UserManager.getInstance(UserManager.java:126)
at com.atlassian.seraph.auth.DefaultAuthenticator.login(DefaultAuthentic
ator.java:64)
at com.atlassian.seraph.filter.LoginFilter.doFilter(LoginFilter.java:129