Started happening recently - actually right after I enabled uca-default collation.
Here's one full backtrace:
[10-Feb-2011 04:08:14] PHP Warning: DOMDocument::loadXML() [<a href='domdocument.loadxml'>domdocument.loadxml</a>]: Empty string supplied as input in /www/w/includes/parser/Prepr
ocessor_DOM.php on line 136
[10-Feb-2011 04:08:14] /wiki/User_talk:Nike/Archive_to_2008-12-06: Exception: Preprocessor_DOM::preprocessToObj generated invalid XML
#0 /www/w/includes/parser/Parser.php(2811): Preprocessor_DOM->preprocessToObj('"??????????????...', 0)
#1 /www/w/includes/parser/Parser.php(2862): Parser->preprocessToDom('"??????????????...')
#2 /www/w/includes/parser/CoreParserFunctions.php(95): Parser->replaceVariables('"??????????????...')
#3 [internal function]: CoreParserFunctions::intFunction(Object(Parser), 'quotes', '???????????????...')
#4 /www/w/includes/parser/Parser.php(3065): call_user_func_array(Array, Array)
#5 /www/w/includes/parser/Preprocessor_DOM.php(988): Parser->braceSubstitution(Array, Object(PPTemplateFrame_DOM))
#6 /www/w/includes/parser/Parser.php(3054): PPFrame_DOM->expand(Object(PPNode_DOM))
#7 /www/w/includes/parser/Preprocessor_DOM.php(988): Parser->braceSubstitution(Array, Object(PPTemplateFrame_DOM))
#8 /www/w/extensions/ParserFunctions/ParserFunctions_body.php(108): PPFrame_DOM->expand(Object(PPNode_DOM))
#9 [internal function]: ExtParserFunctions->ifObj(Object(Parser), Object(PPTemplateFrame_DOM), Array)
#10 /www/w/extensions/ParserFunctions/ParserFunctions.php(127): call_user_func_array(Array, Array)
#11 [internal function]: ParserFunctions_HookStub->call('ifObj', Array)
#12 [internal function]: ParserFunctions_HookStub->ifObj(Object(Parser), Object(PPTemplateFrame_DOM), Array)
#13 /www/w/includes/parser/Parser.php(3065): call_user_func_array(Array, Array)
#14 /www/w/includes/parser/Preprocessor_DOM.php(988): Parser->braceSubstitution(Array, Object(PPTemplateFrame_DOM))
#15 /www/w/extensions/ParserFunctions/ParserFunctions_body.php(385): PPFrame_DOM->expand(Object(PPNode_DOM))
#16 [internal function]: ExtParserFunctions->ifexistObj(Object(Parser), Object(PPTemplateFrame_DOM), Array)
#17 /www/w/extensions/ParserFunctions/ParserFunctions.php(127): call_user_func_array(Array, Array)
#18 [internal function]: ParserFunctions_HookStub->call('ifexistObj', Array)
#19 [internal function]: ParserFunctions_HookStub->ifexistObj(Object(Parser), Object(PPTemplateFrame_DOM), Array)
#20 /www/w/includes/parser/Parser.php(3065): call_user_func_array(Array, Array)
#21 /www/w/includes/parser/Preprocessor_DOM.php(988): Parser->braceSubstitution(Array, Object(PPTemplateFrame_DOM))
#22 /www/w/includes/parser/Parser.php(3198): PPFrame_DOM->expand(Object(PPNode_DOM))
#23 /www/w/includes/parser/Preprocessor_DOM.php(988): Parser->braceSubstitution(Array, Object(PPTemplateFrame_DOM))
#24 /www/w/extensions/ParserFunctions/ParserFunctions_body.php(385): PPFrame_DOM->expand(Object(PPNode_DOM))
#25 [internal function]: ExtParserFunctions->ifexistObj(Object(Parser), Object(PPTemplateFrame_DOM), Array)
#26 /www/w/extensions/ParserFunctions/ParserFunctions.php(127): call_user_func_array(Array, Array)
#27 [internal function]: ParserFunctions_HookStub->call('ifexistObj', Array)
#28 [internal function]: ParserFunctions_HookStub->ifexistObj(Object(Parser), Object(PPTemplateFrame_DOM), Array)
#29 /www/w/includes/parser/Parser.php(3065): call_user_func_array(Array, Array)
#30 /www/w/includes/parser/Preprocessor_DOM.php(988): Parser->braceSubstitution(Array, Object(PPTemplateFrame_DOM))
#31 /www/w/includes/parser/Parser.php(3198): PPFrame_DOM->expand(Object(PPNode_DOM))
#32 /www/w/includes/parser/Preprocessor_DOM.php(988): Parser->braceSubstitution(Array, Object(PPFrame_DOM))
#33 /www/w/includes/parser/Preprocessor_DOM.php(1058): PPFrame_DOM->expand(Object(DOMNodeList), 0)
#34 /www/w/includes/parser/Parser.php(2864): PPFrame_DOM->expand(Object(PPNode_DOM), 0)
#35 /www/w/includes/parser/Parser.php(1025): Parser->replaceVariables('== Translating ...')
#36 /www/w/includes/parser/Parser.php(290): Parser->internalParse('== Translating ...')
#37 /www/w/includes/Article.php(4366): Parser->parse('== Translating ...', Object(Title), Object(ParserOptions), true, true, 890249)
#38 /www/w/includes/Article.php(4566): Article->getOutputFromWikitext('== Translating ...', true)
#39 /www/w/extensions/LiquidThreads/pages/TalkpageView.php(450): Article->getParserOutput()
#40 /www/w/extensions/LiquidThreads/pages/TalkpageView.php(437): LqtDiscussionPager->getPageLimit()
#41 /www/w/extensions/LiquidThreads/pages/TalkpageView.php(379): LqtDiscussionPager->construct(Object(Article), 'nc')
#42 /www/w/extensions/LiquidThreads/pages/TalkpageView.php(322): TalkpageView->getPager()
#43 /www/w/extensions/LiquidThreads/classes/Dispatch.php(54): TalkpageView->show()
#44 /www/w/extensions/LiquidThreads/classes/Dispatch.php(179): LqtDispatch::talkpageMain(Object(OutputPage), Object(Article), Object(Title), Object(User), Object(WebRequest))
#45 [internal function]: LqtDispatch::tryPage(Object(OutputPage), Object(Article), Object(Title), Object(User), Object(WebRequest), Object(MediaWiki))
#46 /www/w/includes/Hooks.php(237): call_user_func_array(Array, Array)
#47 /www/w/includes/Hooks.php(38): Hooks::run('MediaWikiPerfor...', Array)
#48 /www/w/includes/Wiki.php(467): wfRunHooks('MediaWikiPerfor...', Array)
#49 /www/w/includes/Wiki.php(69): MediaWiki->performAction(Object(OutputPage), Object(Article), Object(Title), Object(User), Object(WebRequest))
#50 /www/w/index.php(104): MediaWiki->performRequestForTitle(Object(Title), Object(Article), Object(OutputPage), Object(User), Object(WebRequest))
#51 {main}
Version: 1.18.x
Severity: normal