PT Not Validated

Hi.

I'm trying to set up Drupal as a CAS client and server (using the cas_server module). After attempting to log in on the client side, I get a generic "CAS Authentication failed!" page and I get this in my debugging log:

610 .=> phpCAS::forceAuthentication() [cas.module:83]
5610 .|    => CAS_Client::forceAuthentication() [CAS.php:1081]
5610 .|    |    => CAS_Client::isAuthenticated() [Client.php:962]
5610 .|    |    |    => CAS_Client::wasPreviouslyAuthenticated() [Client.php:1058]
5610 .|    |    |    |    no user found [Client.php:1239]
5610 .|    |    |    <= false
5610 .|    |    |    PT `ST-DPfFPtP9QA' is present [Client.php:1093]
5610 .|    |    |    => CAS_Client::validatePT('', NULL, NULL) [Client.php:1094]
5610 .|    |    |    |     [Client.php:2584]
5610 .|    |    |    |    => CAS_Client::getServerProxyValidateURL() [Client.php:2586]
5610 .|    |    |    |    |    => CAS_Client::getURL() [Client.php:475]
5610 .|    |    |    |    |    <= 'http://d6dev.dimitri.university.edu/cas?destination=node'
5610 .|    |    |    |    <= 'https://playground.dimitri.university.edu/cas/proxyValidate?service=http%3A%2F%2Fd6dev.dimitri.university.edu%2Fcas%3Fdestination%3Dnode'
5610 .|    |    |    |    => CAS_CurlRequest::_sendRequest() [AbstractRequest.php:191]
5610 .|    |    |    |    |    Response Body: 
5610 .|    |    |    |    |    <cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>
5610 .|    |    |    |    |      <cas:authenticationSuccess>
5610 .|    |    |    |    |        <cas:user>admin</cas:user>
5610 .|    |    |    |    |      </cas:authenticationSuccess>
5610 .|    |    |    |    |    </cas:serviceResponse>
5610 .|    |    |    |    |    <div class="dev-query"> Page execution time was 143.99 ms.<span class="dev-memory-usages"> Memory used at: devel_boot()=<strong>3.03</strong> MB, devel_shutdown()=<strong>19.96</strong> MB, PHP peak=<strong>20.5</strong> MB.</span></div>
5610 .|    |    |    |    |     [CurlRequest.php:133]
5610 .|    |    |    |    <= true
5610 .|    |    |    |    => CAS_Client::authError('PT not validated', 'https://playground.dimitri.university.edu/cas/proxyValidate?service=http%3A%2F%2Fd6dev.dimitri.university.edu%2Fcas%3Fdestination%3Dnode&ticket=ST-DPfFPtP9QA', false, true, '<cas:serviceResponse xmlns:cas=\'http://www.yale.edu/tp/cas\'>  <cas:authenticationSuccess>    <cas:user>admin</cas:user>  </cas:authenticationSuccess></cas:serviceResponse><div class="dev-query"> Page execution time was 143.99 ms.<span class="dev-memory-usages"> Memory used at: devel_boot()=<strong>3.03</strong> MB, devel_shutdown()=<strong>19.96</strong> MB, PHP peak=<strong>20.5</strong> MB.</span></div>') [Client.php:2612]
5610 .|    |    |    |    |    => CAS_Client::getURL() [Client.php:3014]
5610 .|    |    |    |    |    <= 'http://d6dev.dimitri.university.edu/cas?destination=node'
5610 .|    |    |    |    |    CAS URL: https://playground.dimitri.university.edu/cas/proxyValidate?service=http%3A%2F%2Fd6dev.dimitri.university.edu%2Fcas%3Fdestination%3Dnode&ticket=ST-DPfFPtP9QA [Client.php:3015]
5610 .|    |    |    |    |    Authentication failure: PT not validated [Client.php:3016]
5610 .|    |    |    |    |    Reason: bad response from the CAS server [Client.php:3021]
5610 .|    |    |    |    |    CAS response: <cas:serviceResponse xmlns:cas='http://www.yale.edu/tp/cas'>
5610 .|    |    |    |    |      <cas:authenticationSuccess>
5610 .|    |    |    |    |        <cas:user>admin</cas:user>
5610 .|    |    |    |    |      </cas:authenticationSuccess>
5610 .|    |    |    |    |    </cas:serviceResponse>
5610 .|    |    |    |    |    <div class="dev-query"> Page execution time was 143.99 ms.<span class="dev-memory-usages"> Memory used at: devel_boot()=<strong>3.03</strong> MB, devel_shutdown()=<strong>19.96</strong> MB, PHP peak=<strong>20.5</strong> MB.</span></div> [Client.php:3035]
5610 .|    |    |    |    |    exit()
5610 .|    |    |    |    |    -
5610 .|    |    |    |    -
5610 .|    |    |    -
5610 .|    |    -
5610 .|    -

I also tried the debugging steps outlined here: http://drupal.org/node/1410198#comment-5489850 but ended up with the following response:

<cas:authenticationFailure code="INVALID_REQUEST">
     Ticket not recognized.
  </cas:authenticationFailure>

Any ideas?