Json exceptions should include the filename of the broken file

Same problem here.

I can't reproduce this on a fresh install with Commerce, Address.
In general it looks like one of the modules you downloaded has a malformed composer.json. You'll need to find out which one
(and we should figure out how to make that more obvious in the future).

You can try enabling errors at admin/config/development/logging and then reloading to see if any extra info is provided.

The ZoneInterface error is unrelated to this module, and points to the Address module being installed without its dependencies (composer drupal install never ran).

The problem occurred when updating an existing RC3 site to D 8.0.0. A new init.php was required. The script "composer Drupal-update" terminates with an exception

[UnexpectedValueException]
Could not decode JSON: Syntax error

Logging provides the following message:

UnexpectedValueException: Could not decode JSON: Syntax error in Drupal\composer_manager\JsonFile::read() (line 36 of modules/contrib/composer_manager/src/JsonFile.php).

Additional log info:

Drupal\composer_manager\PackageManager->getExtensionPackages()
Drupal\composer_manager\PackageManager->buildMergedExtensionPackage()
Drupal\composer_manager\PackageManager->getRequiredPackages()
Drupal\composer_manager\PackageManager->needsComposerUpdate()
composer_manager_requirements('runtime')
call_user_func_array('composer_manager_requirements', Array)
Drupal\Core\Extension\ModuleHandler->invokeAll('requirements', Array)
Drupal\system\SystemManager->listRequirements()
Drupal\system\Controller\SystemInfoController->status()
call_user_func_array(Array, Array)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
call_user_func_array(Object, Array)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)

A new init was required because you accidentally replaced vendor/ and composer.json when you upgraded core.
You're not supposed to do that, only the core folder is supposed to be replaced.

The error is still caused by a module's broken composer.json.
Go through your modules, find out which one it is.
I'll commit a fix so that the error actually shows where it came from.

Done in #2735973: Increase error details on invalid JSON.