We have a DNN site that was previously hosted on a single server, but has been upgraded to a web farm environment that uses replication services.  Server1 and Server2 comprise the webfarm.  Server2 is replicated off Server1.  I ran through the webfarm setup guide, and it pretty much seems to be working - except that something seems to be wrong with the broadcast polling caching provider:  Portals\_default\Log\LogFailures.xml.resources is stacking up a long list of the same error.  Here it is, formatted for easier reading:

<log
  LogGUID="a05a223d-c919-47ba-96d8-1543c0d5d8b8"
  LogTypeKey="GENERAL_EXCEPTION"
  LogUserID="-1"
  LogUserName=""
  LogPortalID="-1"
  LogPortalName=""
  LogCreateDate="2006-08-01T08:56:06.8786248-07:00"
  LogCreateDateNum="0"
  BypassBuffering="false"
  LogServerName="Server1">
 <properties>
  <property>
   <name>AssemblyVersion</name>
   <value>04.00.02</value>
  </property>
  <property>
   <name>Method</name>
   <value>DotNetNuke.Services.Cache.BroadcastPollingCachingProvider.BPCachingProvider.Insert</value>
  </property>
  <property>
   <name>FileName</name>
  </property>
  <property>
   <name>FileLineNumber</name>
   <value>0</value>
  </property>
  <property>
   <name>FileColumnNumber</name>
   <value>0</value>
  </property>
  <property>
   <name>PortalID</name>
   <value>-1</value>
  </property>
  <property>
   <name>PortalName</name>
   <value />
  </property>
  <property>
   <name>UserID</name>
   <value>-1</value>
  </property>
  <property>
   <name>UserName</name>
   <value />
  </property>
  <property>
   <name>ActiveTabID</name>
   <value>-1</value>
  </property>
  <property>
   <name>ActiveTabName</name>
   <value />
  </property>
  <property>
   <name>AbsoluteURL</name>
   <value>/Default.aspx</value>
  </property>
  <property>
   <name>AbsoluteURLReferrer</name>
   <value />
  </property>
  <property>
   <name>ExceptionGUID</name>
   <value>3dd97e6f-3de0-4462-aa79-bddfb9674b36</value>
  </property>
  <property>
   <name>DefaultDataProvider</name>
   <value>DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider</value>
  </property>
  <property>
   <name>InnerException</name>
   <value>Object reference not set to an instance of an object.</value>
  </property>
  <property>
   <name>Message</name>
   <value>System.NullReferenceException: Object reference not set to an instance of an object.
          at DotNetNuke.Services.Cache.BroadcastPollingCachingProvider.BPCachingProvider.Insert(String CacheKey, Object objObject, Boolean PersistAppRestart)
          at DotNetNuke.Framework.Reflection.CreateObject(String TypeName, String CacheKey, Boolean UseCache)
   </value>
  </property>
  <property>
   <name>StackTrace</name>
  </property>
  <property>
   <name>Source</name>
  </property>
 </properties>
</log>

I noticed that the providerpath in the web.config points to a path that does not exist, but so does the providerpath for the file-based caching provider.  Here is that piece of the web.config:

<caching defaultProvider="BroadcastPollingCachingProvider">
      <providers>
        <clear />
        <add name="FileBasedCachingProvider" type="DotNetNuke.Services.Cache.FileBasedCachingProvider.FBCachingProvider, DotNetNuke.Caching.FileBasedCachingProvider" providerPath="~\Providers\CachingProviders\FileBasedCachingProvider\" />
        <add name="BroadcastPollingCachingProvider" type="DotNetNuke.Services.Cache.BroadcastPollingCachingProvider.BPCachingProvider, DotNetNuke.Caching.BroadcastPollingCachingProvider" providerPath="~\Providers\CachingProviders\BroadcastPollingCachingProvider\" />
      </providers>
    </caching>

Any ideas what could be causing this?  This is a fresh install, and otherwise very stable.  Could not find anything on Google.