After updating Mediawiki to v1.26.3 we get the following error:
PHP Fatal error: Uncaught exception 'InvalidArgumentException' with message 'The value for 'load_composer_autoloader' should be an array' in /home/khronos/public_html/webgl/wiki_1_15/includes/registration/ExtensionProcessor.php:348 Stack trace: #0 /home/khronos/public_html/webgl/wiki_1_15/includes/registration/ExtensionProcessor.php(173): ExtensionProcessor->storeToArray('load_composer_a...', true, Array) #1 /home/khronos/public_html/webgl/wiki_1_15/includes/registration/ExtensionRegistry.php(202): ExtensionProcessor->extractInfo('/home/khronos/p...', Array, 1) #2 /home/khronos/public_html/webgl/wiki_1_15/includes/registration/ExtensionRegistry.php(127): ExtensionRegistry->readFromQueue(Array) #3 /home/khronos/public_html/webgl/wiki_1_15/includes/Setup.php(39): ExtensionRegistry->loadFromQueue() #4 /home/khronos/public_html/webgl/wiki_1_15/includes/WebStart.php(140): require_once('/home/khronos/p...') #5 /home/khronos/public_html/webgl/wiki_1_15/index.php(38): require('/home/khronos/p...') #6 {main}
Commenting out load_composer_autoloader fixes the issue.