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PHP Warning:  call_user_func() expects parameter 1 to be a valid callback, cannot access private method BackupReader::reportPage() in /var/www/mediawiki/includes/import/WikiImporter.php on line 474

PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, cannot access private method BackupReader::handleRevision() in /var/www/mediawiki/includes/import/WikiImporter.php on line 500

PHP Warning:  call_user_func() expects parameter 1 to be a valid callback, cannot access private method BackupReader::reportPage() in /var/www/mediawiki/includes/import/WikiImporter.php on line 474

PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, cannot access private method BackupReader::handleRevision() in /var/www/mediawiki/includes/import/WikiImporter.php on line 500

PHP Warning:  call_user_func() expects parameter 1 to be a valid callback, cannot access private method BackupReader::reportPage() in /var/www/mediawiki/includes/import/WikiImporter.php on line 474

PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, cannot access private method BackupReader::handleRevision() in /var/www/mediawiki/includes/import/WikiImporter.php on line 500

PHP Warning:  call_user_func() expects parameter 1 to be a valid callback, cannot access private method BackupReader::reportPage() in /var/www/mediawiki/includes/import/WikiImporter.php on line 474

PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, cannot access private method BackupReader::handleRevision() in /var/www/mediawiki/includes/import/WikiImporter.php on line 500

PHP Warning:  call_user_func() expects parameter 1 to be a valid callback, cannot access private method BackupReader::reportPage() in /var/www/mediawiki/includes/import/WikiImporter.php on line 474

PHP Warning:  call_user_func_array() expects parameter 1 to be a valid callback, cannot access private method BackupReader::handleRevision() in /var/www/mediawiki/includes/import/WikiImporter.php on line 500

PHP Warning:  call_user_func() expects parameter 1 to be a valid callback, cannot access private method BackupReader::reportPage() in /var/www/mediawiki/includes/import/WikiImporter.php on line 474

MWUnknownContentModelException from line 267 of /var/www/mediawiki/includes/content/ContentHandler.php: The content model 'flow-board' is not registered on this wiki.

See https://www.mediawiki.org/wiki/Content_handlers to find out which extensions handle this content model.

#0 /var/www/mediawiki/includes/import/WikiRevision.php(444): ContentHandler::getForModelID('flow-board')

#1 /var/www/mediawiki/includes/import/WikiImporter.php(911): WikiRevision->getContentHandler()

#2 /var/www/mediawiki/includes/import/WikiImporter.php(860): WikiImporter->processRevision(Array, Array)

#3 /var/www/mediawiki/includes/import/WikiImporter.php(802): WikiImporter->handleRevision(Array)

#4 /var/www/mediawiki/includes/import/WikiImporter.php(606): WikiImporter->handlePage()

#5 /var/www/mediawiki/maintenance/importDump.php(359): WikiImporter->doImport()

#6 /var/www/mediawiki/maintenance/importDump.php(301): BackupReader->importFromHandle(Resource id #174)

#7 /var/www/mediawiki/maintenance/importDump.php(129): BackupReader->importFromStdin()

#8 /var/www/mediawiki/maintenance/doMaintenance.php(99): BackupReader->execute()

#9 /var/www/mediawiki/maintenance/importDump.php(364): require_once('/var/www/mediaw...')

#10 {main}

This seems to be phab:T238984, which hasn't been backported to 1.34. Can you confirm which MediaWiki version are you using?

mediawiki version 1.34

is there any way to solve this error?

You don't actually seem to be using MediaWiki 1.34, you seem to be using master or a 1.35 WMF branch, as the code that caused this issue is not in 1.34. There is nothing to be backported

Can you check again, and if you installed MediaWiki via git, try running git pull to get the latest code?