Improper definition of exposedHeaders in sites/default/default.services.yml causes the following watchdog error.
Warning: implode(): Invalid arguments passed in Asm89\Stack\CorsService->addActualRequestHeaders() (line 85 of /my/drupal/vendor/asm89/stack-cors/src/Asm89/Stack/CorsService.php)
#0 /my/drupal/core/includes/bootstrap.inc(566): _drupal_error_handler_real(2, 'implode(): Inva...', '/my/drupal/...', 85, Array)
#1 [internal function]: _drupal_error_handler(2, 'implode(): Inva...', '/my/drupal/...', 85, Array)
#2 /my/drupal/vendor/asm89/stack-cors/src/Asm89/Stack/CorsService.php(85): implode(', ', true)
#3 /my/drupal/vendor/asm89/stack-cors/src/Asm89/Stack/Cors.php(53): Asm89\Stack\CorsService->addActualRequestHeaders(Object(Symfony\Component\HttpFoundation\JsonResponse), Object(Symfony\Component\HttpFoundation\Request))
#4 /my/drupal/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Asm89\Stack\Cors->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#5 /my/drupal/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(50): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#6 /my/drupal/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#7 /my/drupal/core/lib/Drupal/Core/DrupalKernel.php(656): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true)
#8 /my/drupal/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #9 {main}.
This is due to the fact that asm89/stack-cors expects exposedHeaders to either be false, or an array. If you end up setting this to true it tries to implode a boolean and throws an error.