As I said before the backtrace is really ambiguous therefore one can't really tell where, when, and how things are happening.
I also tried to log ($wgDebugLogFile) any other possible messages but the log file does not show any related information to the above problem.
Maybe some wfDebugLog( 'SolrStore', __METHOD__,... log messages could help to shed light on where it comes to problems. Using the this message type would allow to filter all related Solr message using $wgDebugLogGroups = ...
Anyway the last backtrace based on SVN r114866.
The request sent by the client was syntactically incorrect (unexpected XML tag d
oc/span).
Backtrace:
#0 D:\xampp\htdocs\...\extensions\SolrStore\SolrTalker.php(211): SolrTalker->so
lrSend('http://192.168....', '<add><doc><fiel...')
#1 D:\xampp\htdocs\...\extensions\SolrStore\SolrTalker.php(281): SolrTalker->so
lrAdd('<add><doc><fiel...')
#2 D:\xampp\htdocs\...\extensions\SolrStore\SolrTalker.php(441): SolrTalker->ad
dDoc(Object(SolrDoc))
#3 D:\xampp\htdocs\...\extensions\SolrStore\SolrConnectorStore.php(139): SolrTa
lker->parseSemanticData(Object(SMWSemanticData))
#4 D:\xampp\htdocs\...\extensions\SemanticMediaWiki\includes\storage\SMW_Store.
php(303): SolrConnectorStore->doDataUpdate(Object(SMWSemanticData))
#5 D:\xampp\htdocs\...\extensions\SemanticMediaWiki\includes\SMW_ParseData.php(
316): SMWStore->updateData(Object(SMWSemanticData))
#6 D:\xampp\htdocs\...\extensions\SemanticMediaWiki\includes\SMW_ParseData.php(
445): SMWParseData::storeData(Object(ParserOutput), Object(Title), true)
#7 [internal function]: SMWParseData::onLinksUpdateConstructed(Object(LinksUpdat
e))
#8 D:\xampp\htdocs\...\includes\Hooks.php(216): call_user_func_array('SMWParseD
ata::o...', Array)
#9 D:\xampp\htdocs\...\includes\GlobalFunctions.php(3631): Hooks::run('LinksUpd
ateCons...', Array)
#10 D:\xampp\htdocs\...\includes\LinksUpdate.php(98): wfRunHooks('LinksUpdateCo
ns...', Array)
#11 D:\xampp\htdocs\...\includes\job\RefreshLinksJob.php(49): LinksUpdate->__co
nstruct(Object(Title), Object(ParserOutput), false)
#12 D:\xampp\htdocs\...\maintenance\runJobs.php(78): RefreshLinksJob->run()
#13 D:\xampp\htdocs\...\maintenance\doMaintenance.php(105): RunJobs->execute()
#14 D:\xampp\htdocs\...\maintenance\runJobs.php(108): require_once('D:\xampp\ht
docs...')
#15 {main}