MediaWiki internal error.

Original exception: [d0b9cf80d943d38b9080038d] /hcewiki/index.php?title=Special:CiteThisPage&page=Valedo&id=1897&mobileaction=toggle_view_mobile RuntimeException from line 61 of /var/www/mediawiki/extensions/MobileFrontend/includes/MobileFrontend.hooks.php: wgMFDefaultSkinClass is not setup correctly. It should point to the class name of a valid skin e.g. SkinMinerva, SkinVector
Backtrace:
#0 /var/www/mediawiki/extensions/MobileFrontend/includes/MobileFrontend.hooks.php(141): MobileFrontendHooks::getDefaultMobileSkin(RequestContext, MobileContext)
#1 /var/www/mediawiki/includes/Hooks.php(174): MobileFrontendHooks::onRequestContextCreateSkin(RequestContext, NULL)
#2 /var/www/mediawiki/includes/Hooks.php(202): Hooks::callHook(string, array, array, NULL)
#3 /var/www/mediawiki/includes/context/RequestContext.php(370): Hooks::run(string, array)
#4 /var/www/mediawiki/includes/context/ContextSource.php(137): RequestContext->getSkin()
#5 /var/www/mediawiki/includes/OutputPage.php(4232): ContextSource->getSkin()
#6 /var/www/mediawiki/includes/htmlform/OOUIHTMLForm.php(33): OutputPage->enableOOUI()
#7 /var/www/mediawiki/vendor/wikimedia/object-factory/src/ObjectFactory.php(156): OOUIHTMLForm->__construct(array, RequestContext, string)
#8 /var/www/mediawiki/includes/htmlform/HTMLForm.php(294): Wikimedia\ObjectFactory::constructClassInstance(string, array)
#9 /var/www/mediawiki/includes/specialpage/FormSpecialPage.php(120): HTMLForm::factory(string, array, RequestContext, string)
#10 /var/www/mediawiki/includes/specialpage/FormSpecialPage.php(183): FormSpecialPage->getForm()
#11 /var/www/mediawiki/extensions/CiteThisPage/includes/SpecialCiteThisPage.php(24): FormSpecialPage->execute(NULL)
#12 /var/www/mediawiki/includes/specialpage/SpecialPage.php(569): SpecialCiteThisPage->execute(NULL)
#13 /var/www/mediawiki/includes/specialpage/SpecialPageFactory.php(558): SpecialPage->run(NULL)
#14 /var/www/mediawiki/includes/MediaWiki.php(288): MediaWiki\Special\SpecialPageFactory->executePath(Title, RequestContext)
#15 /var/www/mediawiki/includes/MediaWiki.php(865): MediaWiki->performRequest()
#16 /var/www/mediawiki/includes/MediaWiki.php(515): MediaWiki->main()
#17 /var/www/mediawiki/index.php(42): MediaWiki->run()
#18 {main}

Exception caught inside exception handler: [d0b9cf80d943d38b9080038d] /hcewiki/index.php?title=Special:CiteThisPage&page=Valedo&id=1897&mobileaction=toggle_view_mobile RuntimeException from line 61 of /var/www/mediawiki/extensions/MobileFrontend/includes/MobileFrontend.hooks.php: wgMFDefaultSkinClass is not setup correctly. It should point to the class name of a valid skin e.g. SkinMinerva, SkinVector
Backtrace:
#0 /var/www/mediawiki/extensions/MobileFrontend/includes/MobileFrontend.hooks.php(141): MobileFrontendHooks::getDefaultMobileSkin(RequestContext, MobileContext)
#1 /var/www/mediawiki/includes/Hooks.php(174): MobileFrontendHooks::onRequestContextCreateSkin(RequestContext, NULL)
#2 /var/www/mediawiki/includes/Hooks.php(202): Hooks::callHook(string, array, array, NULL)
#3 /var/www/mediawiki/includes/context/RequestContext.php(370): Hooks::run(string, array)
#4 /var/www/mediawiki/includes/context/ContextSource.php(137): RequestContext->getSkin()
#5 /var/www/mediawiki/includes/OutputPage.php(2682): ContextSource->getSkin()
#6 /var/www/mediawiki/includes/exception/MWExceptionRenderer.php(134): OutputPage->output()
#7 /var/www/mediawiki/includes/exception/MWExceptionRenderer.php(53): MWExceptionRenderer::reportHTML(RuntimeException)
#8 /var/www/mediawiki/includes/exception/MWExceptionHandler.php(98): MWExceptionRenderer::output(RuntimeException, integer)
#9 /var/www/mediawiki/includes/exception/MWExceptionHandler.php(172): MWExceptionHandler::report(RuntimeException)
#10 /var/www/mediawiki/includes/MediaWiki.php(542): MWExceptionHandler::handleException(RuntimeException)
#11 /var/www/mediawiki/index.php(42): MediaWiki->run()
#12 {main}