Ok, so this is what it throws when enabling exception details:
Exception encountered, of type "Error"
[XjhNg6So5dm0pad9ZKwy-QAAAAY] /mediawiki/load.php?debug=false&lang=en&modules=startup&only=scripts&skin=modern Error from line 663 of /var/www/html/mediawiki/includes/exception/MWExceptionHandler.php: Class 'FormatJson' not found
Backtrace:
#0 /var/www/html/mediawiki/includes/exception/MWExceptionHandler.php(242): MWExceptionHandler::logError(ErrorException, string)
#1 /var/www/html/mediawiki/includes/AutoLoader.php(81): MWExceptionHandler::handleError(integer, string, string, integer, array)
#2 /var/www/html/mediawiki/includes/AutoLoader.php(81): require()
#3 [internal function]: AutoLoader::autoload(string)
#4 /var/www/html/mediawiki/includes/cache/MessageBlobStore.php(243): spl_autoload_call(string)
#5 /var/www/html/mediawiki/includes/cache/MessageBlobStore.php(163): MessageBlobStore->generateMessageBlob(ResourceLoaderFileModule, string)
#6 /var/www/html/mediawiki/includes/cache/MessageBlobStore.php(116): MessageBlobStore->recacheMessageBlob(string, ResourceLoaderFileModule, string)
#7 /var/www/html/mediawiki/includes/resourceloader/ResourceLoader.php(152): MessageBlobStore->getBlobs(array, string)
#8 /var/www/html/mediawiki/includes/resourceloader/ResourceLoaderStartUpModule.php(397): ResourceLoader->preloadModuleInfo(array, DerivativeResourceLoaderContext)
#9 /var/www/html/mediawiki/includes/resourceloader/ResourceLoaderStartUpModule.php(379): ResourceLoaderStartUpModule->getAllModuleHashes(DerivativeResourceLoaderContext)
#10 /var/www/html/mediawiki/includes/resourceloader/ResourceLoaderModule.php(707): ResourceLoaderStartUpModule->getDefinitionSummary(DerivativeResourceLoaderContext)
#11 /var/www/html/mediawiki/includes/resourceloader/ResourceLoader.php(622): ResourceLoaderModule->getVersionHash(DerivativeResourceLoaderContext)
#12 [internal function]: ResourceLoader->{closure}(string)
#13 /var/www/html/mediawiki/includes/resourceloader/ResourceLoader.php(623): array_map(Closure, array)
#14 /var/www/html/mediawiki/includes/resourceloader/ResourceLoader.php(675): ResourceLoader->getCombinedVersion(ResourceLoaderContext, array)
#15 /var/www/html/mediawiki/load.php(47): ResourceLoader->respond(ResourceLoaderContext)
#16 {main}
I tried verifying JSON module is active on my PHP 7.3 installation and it is. However the error is not fixed yet.
Remember my version of mediawiki is 1.27, there might be incompatibility issues with this version?