Endless redirect loop after ticket validation - validation

Question

I'm using the Liferay Portal 6.1.1 CE GA2.
After hours of research I got the following things to work:
Import from a LDAP-Server and custom mapping of an attribute to an usergroup.
Redirect to a specific page after login (based on the usergroup).
Authentication via CAS. This means getting a ServiceTicket and logging in the corresponding user.
Now I'm trying to obtain ProxyTickets so I can proxy to other applications behind the same CAS-Server.
I'm not really getting any error, but Mozilla gives me a redirection error, e.g. the page is redirected in such a way that it can never be loaded.
I googled a lot and tried different approaches but nothing helped.
My web.xml is configured as follows (I snipped out urls. If they're important I can hand them in later):
<filter>
<filter-name>CAS Authentication Filter</filter-name>
<filter-class>org.jasig.cas.client.authentication.AuthenticationFilter</filter-class>
<init-param>
<param-name>casServerLoginUrl</param-name>
<param-value>* snip */cas/login</param-value>
</init-param>
<init-param>
<param-name>serverName</param-name>
<param-value>* snip *</param-value>
</init-param>
</filter>
<filter>
<filter-name>CAS Validation Filter</filter-name>
<filter-class>org.jasig.cas.client.validation.Cas20ProxyReceivingTicketValidationFilter</filter-class>
<init-param>
<param-name>casServerUrlPrefix</param-name>
<param-value>* snip */cas/</param-value>
</init-param>
<init-param>
<param-name>serverName</param-name>
<param-value>* snip *</param-value>
</init-param>
<init-param>
<param-name>proxyCallbackUrl</param-name>
<param-value>https://* snip */pgtCallback</param-value>
</init-param>
<init-param>
<param-name>proxyReceptorUrl</param-name>
<param-value>/pgtCallback</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>CAS Validation Filter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
<filter-mapping>
<filter-name>CAS Authentication Filter</filter-name>
<url-pattern>/c/portal/login</url-pattern>
</filter-mapping>
I tried various combinations of the filter-mappings but nothing helped.
The output of the console in Eclipse hints that multiple consecutive requests are done. Each gets me a TGT, PGTIOU and PGT but after the ST is validated a new validation request is fired. This goes until Mozilla ends the redirect loop.
I also tried specifying service instead of serverName but all remains the same.
Setting the param redirectAfterValidation to false but then I get a MalformedURLException.
Hopefully I didn't forget any information, please help me.
Thanks in advance.