Drupal 6.x Known issue:

Looking at solrconfig_extra.xml you will find stuff like this:

<str name="accuracy">0.5</str>
<str name="maxEdits">2</str>
<str name="minPrefix">1</str>
<str name="maxInspections">5</str>
<str name="minQueryLength">4</str>
<str name="maxQueryFrequency">0.01</str>
<str name="thresholdTokenFrequency">.01</str>

You will have to change it to stuff like this:

<int name="maxEdits">2</int>
<int name="minPrefix">1</int>
<int name="maxInspections">5</int>
<int name="minQueryLength">4</int>
<float name="maxQueryFrequency">0.01</float>
<float name="thresholdTokenFrequency">.01</float>

I hope it's obvious why...
And please  try to find more occurrences of integers and floats being defined as strings, in the drupal config files... I'm sure there must be more erors like these, in other configs for other solr versions as well in drupal.