MediaWiki internal error.

Original exception: [ZgWWaPkl0Xs@IYPhvmK@9AAB0Qg] /index.php?title=Speciaal:Aanmelden&returnto=De+letter+N&mobileaction=toggle_view_mobile RuntimeException from line 61 of /var/www/clients/client1/web159/web/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/clients/client1/web159/web/extensions/MobileFrontend/includes/MobileFrontend.hooks.php(141): MobileFrontendHooks::getDefaultMobileSkin()
#1 /var/www/clients/client1/web159/web/includes/Hooks.php(174): MobileFrontendHooks::onRequestContextCreateSkin()
#2 /var/www/clients/client1/web159/web/includes/Hooks.php(202): Hooks::callHook()
#3 /var/www/clients/client1/web159/web/includes/context/RequestContext.php(370): Hooks::run()
#4 /var/www/clients/client1/web159/web/includes/context/ContextSource.php(137): RequestContext->getSkin()
#5 /var/www/clients/client1/web159/web/includes/OutputPage.php(2682): ContextSource->getSkin()
#6 /var/www/clients/client1/web159/web/includes/MediaWiki.php(872): OutputPage->output()
#7 /var/www/clients/client1/web159/web/includes/MediaWiki.php(884): MediaWiki->{closure}()
#8 /var/www/clients/client1/web159/web/includes/MediaWiki.php(515): MediaWiki->main()
#9 /var/www/clients/client1/web159/web/index.php(42): MediaWiki->run()
#10 {main}

Exception caught inside exception handler: [ZgWWaPkl0Xs@IYPhvmK@9AAB0Qg] /index.php?title=Speciaal:Aanmelden&returnto=De+letter+N&mobileaction=toggle_view_mobile RuntimeException from line 61 of /var/www/clients/client1/web159/web/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/clients/client1/web159/web/extensions/MobileFrontend/includes/MobileFrontend.hooks.php(141): MobileFrontendHooks::getDefaultMobileSkin()
#1 /var/www/clients/client1/web159/web/includes/Hooks.php(174): MobileFrontendHooks::onRequestContextCreateSkin()
#2 /var/www/clients/client1/web159/web/includes/Hooks.php(202): Hooks::callHook()
#3 /var/www/clients/client1/web159/web/includes/context/RequestContext.php(370): Hooks::run()
#4 /var/www/clients/client1/web159/web/includes/context/ContextSource.php(137): RequestContext->getSkin()
#5 /var/www/clients/client1/web159/web/includes/OutputPage.php(2682): ContextSource->getSkin()
#6 /var/www/clients/client1/web159/web/includes/exception/MWExceptionRenderer.php(134): OutputPage->output()
#7 /var/www/clients/client1/web159/web/includes/exception/MWExceptionRenderer.php(53): MWExceptionRenderer::reportHTML()
#8 /var/www/clients/client1/web159/web/includes/exception/MWExceptionHandler.php(98): MWExceptionRenderer::output()
#9 /var/www/clients/client1/web159/web/includes/exception/MWExceptionHandler.php(172): MWExceptionHandler::report()
#10 /var/www/clients/client1/web159/web/includes/MediaWiki.php(542): MWExceptionHandler::handleException()
#11 /var/www/clients/client1/web159/web/index.php(42): MediaWiki->run()
#12 {main}