When the web-based installer is used, a default main page is created. When later this becomes a Wikibase repo, this main page "thinks" it is an item.
The error message should make the user notice this. Instead, it says something hard to debug...
failed to deserialize
Backtrace:
#0 /srv/testrepo/w/extensions/Wikibase/lib/includes/entity/EntityFactory.php(139): Wikibase\EntityFactory->unserializedData(''''MediaWiki ha...', 'application/jso...')
#1 /srv/testrepo/w/extensions/Wikibase/repo/includes/content/ItemHandler.php(86): Wikibase\EntityFactory->newFromBlob('item', ''''MediaWiki ha...', 'application/jso...')
#2 /srv/testrepo/w/includes/Revision.php(1005): Wikibase\ItemHandler->unserializeContent(''''MediaWiki ha...', 'application/jso...')
#3 /srv/testrepo/w/includes/Revision.php(946): Revision->getContentInternal()
#4 /srv/testrepo/w/includes/WikiPage.php(616): Revision->getContent(1)
#5 /srv/testrepo/w/includes/WikiPage.php(448): WikiPage->getContent()
#6 [internal function]: WikiPage->isRedirect()
#7 /srv/testrepo/w/includes/Article.php(1944): call_user_func_array(Array, Array)
#8 /srv/testrepo/w/includes/Wiki.php(364): Article->__call('isRedirect', Array)
#9 /srv/testrepo/w/includes/Wiki.php(364): Article->isRedirect()
#10 /srv/testrepo/w/includes/Wiki.php(295): MediaWiki->initializeArticle()
#11 /srv/testrepo/w/includes/Wiki.php(555): MediaWiki->performRequest()
#12 /srv/testrepo/w/includes/Wiki.php(448): MediaWiki->main()
#13 /srv/testrepo/w/index.php(59): MediaWiki->run()
#14 {main}
Version: unspecified
Severity: minor