This occurs when called from CirrusSearch's forceSearchIndex.php. The result, after indexing some pages but before finishing the job, is:
MWException from line 348 of /var/www/mediawiki-1.34.0/includes/parser/ParserOutput.php: Bad parser output text.
#0 [internal function]: ParserOutput->{closure}(Array)
#1 /var/www/mediawiki-1.34.0/includes/parser/ParserOutput.php(359): preg_replace_callback('#<(?:mw:)?edits...', Object(Closure), '<div class="mw-...')
#2 /var/www/mediawiki-1.34.0/includes/content/WikiTextStructure.php(154): ParserOutput->getText(Array)
#3 /var/www/mediawiki-1.34.0/includes/content/WikiTextStructure.php(223): WikiTextStructure->extractWikitextParts()
#4 /var/www/mediawiki-1.34.0/includes/content/WikitextContentHandler.php(152): WikiTextStructure->getOpeningText()
#5 /var/www/mediawiki-1.34.0/extensions/CirrusSearch/includes/Updater.php(380): WikitextContentHandler->getDataForSearchIndex(Object(WikiPage), Object(ParserOutput), Object(CirrusSearch\CirrusSearch))
#6 /var/www/mediawiki-1.34.0/extensions/CirrusSearch/includes/Updater.php(458): CirrusSearch\Updater::buildDocument(Object(CirrusSearch\CirrusSearch), Object(WikiPage), Object(CirrusSearch\Connection), 0, 0, 0)
#7 /var/www/mediawiki-1.34.0/extensions/CirrusSearch/includes/Updater.php(236): CirrusSearch\Updater->buildDocumentsForPages(Array, 0)
#8 /var/www/mediawiki-1.34.0/extensions/CirrusSearch/maintenance/forceSearchIndex.php(219): CirrusSearch\Updater->updatePages(Array, 0)
#9 /var/www/mediawiki-1.34.0/maintenance/doMaintenance.php(99): CirrusSearch\ForceSearchIndex->execute()
#10 /var/www/mediawiki-1.34.0/extensions/CirrusSearch/maintenance/forceSearchIndex.php(689): require_once('/var/www/mediaw...')
#11 {main}
This is with CirrusSearch-REL1_34-a86e0a5.tar.gz
Subsequently added an "echo" to see what it was choking on, which looks to be this:
<h2><span class="mw-headline" id="Links">Links</span><mw:editsection page="File::Spec" section="1">Links</mw:editsection></h2>
<ul><li><a rel="nofollow" class="external free" href="http://perldoc.perl.org/File/Spec.html">http://perldoc.perl.org/File/Spec.html</a></li></ul>
On the older version of the wiki, searching for perldoc (in SphinxSearch in this case brings up just this:
{F31554958}
Clicking on which shows this error:
{F31554962}
In other words this appears to be about something that can only be inherited from really old content, but which might be more gracefully skipped over when encountered.