After attempting to update to MediaWiki 1.38, I also updated the Wikibase extension. Nevertheless, both the new and the old Wikibase are currently producing the same error, hinting at a problem with how Wikibase interacts with the new MediaWiki version 1.38.
Full error log:
[5fd0ce49dfdffcca08f1df96] /wiki/ProleWiki:Main_page Error: Class 'DataValues\Deserializers\DataValueDeserializer' not found Backtrace: from /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(264) #0 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}() #1 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService() #2 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService() #3 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService() #4 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(361): Wikimedia\Services\ServiceContainer->get() #5 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(860): Wikibase\Client\WikibaseClient::getDataValueDeserializer() #6 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}() #7 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService() #8 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService() #9 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService() #10 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(155): Wikimedia\Services\ServiceContainer->get() #11 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(1057): Wikibase\Client\WikibaseClient::getSingleEntitySourceServicesFactory() #12 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}() #13 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService() #14 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService() #15 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService() #16 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(160): Wikimedia\Services\ServiceContainer->get() #17 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(956): Wikibase\Client\WikibaseClient::getWikibaseServices() #18 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}() #19 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService() #20 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService() #21 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService() #22 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(222): Wikimedia\Services\ServiceContainer->get() #23 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(401): Wikibase\Client\WikibaseClient::getStore() #24 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}() #25 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService() #26 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService() #27 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService() #28 /var/www/prole/extensions/Wikibase/client/includes/WikibaseClient.php(170): Wikimedia\Services\ServiceContainer->get() #29 /var/www/prole/extensions/Wikibase/client/WikibaseClient.ServiceWiring.php(826): Wikibase\Client\WikibaseClient::getEntityLookup() #30 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(447): Wikimedia\Services\ServiceContainer->{closure}() #31 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(416): Wikimedia\Services\ServiceContainer->createService() #32 /var/www/prole/includes/MediaWikiServices.php(294): Wikimedia\Services\ServiceContainer->getService() #33 /var/www/prole/vendor/wikimedia/services/src/ServiceContainer.php(424): MediaWiki\MediaWikiServices->getService() #34 /var/www/prole/vendor/wikimedia/object-factory/src/ObjectFactory.php(211): Wikimedia\Services\ServiceContainer->get() #35 /var/www/prole/vendor/wikimedia/object-factory/src/ObjectFactory.php(152): Wikimedia\ObjectFactory\ObjectFactory::getObjectFromSpec() #36 /var/www/prole/includes/HookContainer/HookContainer.php(473): Wikimedia\ObjectFactory\ObjectFactory->createObject() #37 /var/www/prole/includes/HookContainer/HookContainer.php(156): MediaWiki\HookContainer\HookContainer->getHandlers() #38 /var/www/prole/includes/HookContainer/HookRunner.php(2858): MediaWiki\HookContainer\HookContainer->run() #39 /var/www/prole/includes/parser/Parser.php(635): MediaWiki\HookContainer\HookRunner->onParserClearState() #40 /var/www/prole/includes/parser/Parser.php(4833): Parser->clearState() #41 /var/www/prole/includes/parser/Parser.php(926): Parser->startParse() #42 /var/www/prole/includes/parser/Parser.php(4860): Parser->preprocess() #43 /var/www/prole/includes/cache/MessageCache.php(1257): Parser->transformMsg() #44 /var/www/prole/includes/language/Message.php(1474): MessageCache->transform() #45 /var/www/prole/includes/language/Message.php(1007): Message->transformText() #46 /var/www/prole/includes/language/Message.php(1071): Message->format() #47 /var/www/prole/includes/OutputPage.php(1041): Message->text() #48 /var/www/prole/includes/OutputPage.php(1090): OutputPage->setHTMLTitle() #49 /var/www/prole/includes/page/Article.php(464): OutputPage->setPageTitle() #50 /var/www/prole/includes/actions/ViewAction.php(80): Article->view() #51 /var/www/prole/includes/MediaWiki.php(543): ViewAction->show() #52 /var/www/prole/includes/MediaWiki.php(321): MediaWiki->performAction() #53 /var/www/prole/includes/MediaWiki.php(912): MediaWiki->performRequest() #54 /var/www/prole/includes/MediaWiki.php(563): MediaWiki->main() #55 /var/www/prole/index.php(53): MediaWiki->run() #56 /var/www/prole/index.php(46): wfIndexMain() #57 {main}