"pysolr.SolrError: [Reason: /solr4/update/]" when running mongo_connector.py - mongodb

Question

As a follow on from this problem I was having before: (How long does mongo_connector.py usually take?)
I was wondering if anyone else has had this problem when running the following:
$ python /usr/local/lib/python2.7/dist-packages/mongo-connector/mongo_connector.py -m localhost:27017 --docManager /usr/local/lib/python2.7/dist-packages/mongo-connector/doc_managers/solr_doc_manager.py -t http://localhost:8080/solr4
This is the error output I get:
2012-08-20 10:24:11,893 - INFO - Beginning Mongo Connector
2012-08-20 10:24:12,971 - INFO - Starting new HTTP connection (1): localhost
2012-08-20 10:24:12,974 - INFO - Finished 'http://localhost:8080/solr4/update/?commit=true' (post) with body 'u'<commit ' in 0.017 seconds.
2012-08-20 10:24:12,983 - ERROR - [Reason: /solr4/update/]
Traceback (most recent call last):
File "/usr/local/lib/python2.7/dist-packages/mongo-connector/mongo_connector.py", line 441, in <module>
auth_username=options.admin_name)
File "/usr/local/lib/python2.7/dist-packages/mongo-connector/mongo_connector.py", line 100, in __init__
unique_key=u_key)
File "/usr/local/lib/python2.7/dist-packages/mongo-connector/doc_managers/solr_doc_manager.py", line 54, in __init__
self.run_auto_commit()
File "/usr/local/lib/python2.7/dist-packages/mongo-connector/doc_managers/solr_doc_manager.py", line 95, in run_auto_commit
self.solr.commit()
File "/usr/local/lib/python2.7/dist-packages/pysolr.py", line 802, in commit
return self._update(msg, waitFlush=waitFlush, waitSearcher=waitSearcher)
File "/usr/local/lib/python2.7/dist-packages/pysolr.py", line 359, in _update
return self._send_request('post', path, message, {'Content-type': 'text/xml; charset=utf-8'})
File "/usr/local/lib/python2.7/dist-packages/pysolr.py", line 293, in _send_request
raise SolrError(error_message)
pysolr.SolrError: [Reason: /solr4/update/]
Reason: [Reason: /solr4/update/] is not really an output that I can even start to debug. Solr is working perfectly fine, MongoDB is working perfectly fine. What could this problem be caused by?
I have been following the instructions on this page up to now: http://loutilities.wordpress.com/2012/11/26/complementing-mongodb-with-real-time-solr-search/#comment-183. I've also seen on various websites that adding the following to my Solr's solrconfig.xml should make 'update' accessible, but this is already configured on my system:
<requestHandler name="/update" class="solr.XmlUpdateRequestHandler">
That's about all the information I have. Any hints as to what I might be doing wrong?