Why an exception is raised when I insert a token as a value of an html src attribute

How to raise the exception:

In any text_area field using ckeditor, insert the following code <img src="[node:my_custom_token]"> and save the form.

You will get the following error: (assuming the edited form was http://mysite/fr/node/8/edit)

page not found|127.0.0.1|http://mysite/fr/node/8/[node:my_custom_token]

This is a NotFoundHttpException thrown in RouterListener

Here is the trace

PHP internal call Symfony\Component\HttpKernel\EventListener\RouterListener->onKernelRequest(Symfony\Component\HttpKernel\Event\GetResponseEvent $event)
<ROOT>/core/lib/Drupal/Component/EventDispatcher/ContainerAwareEventDispatcher.php:142 call_user_func($function_name, $parameters)
<ROOT>/vendor/symfony/http-kernel/HttpKernel.php:134 Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch($event)
<ROOT>/vendor/symfony/http-kernel/HttpKernel.php:80 Symfony\Component\HttpKernel\HttpKernel->handleRaw(Symfony\Component\HttpFoundation\Request $request, int $type = 1)
<ROOT>/core/lib/Drupal/Core/StackMiddleware/Session.php:57 Symfony\Component\HttpKernel\HttpKernel->handle(Symfony\Component\HttpFoundation\Request $request, $type = 1, $catch = true)
<ROOT>/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php:47 Drupal\Core\StackMiddleware\Session->handle(Symfony\Component\HttpFoundation\Request $request, $type = 1, $catch = true)
<ROOT>/core/modules/page_cache/src/StackMiddleware/PageCache.php:106 Drupal\Core\StackMiddleware\KernelPreHandle->handle(Symfony\Component\HttpFoundation\Request $request, $type = 1, $catch = true)
<ROOT>/core/modules/page_cache/src/StackMiddleware/PageCache.php:85 Drupal\page_cache\StackMiddleware\PageCache->pass(Symfony\Component\HttpFoundation\Request $request, $type = 1, $catch = true)
<ROOT>/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php:47 Drupal\page_cache\StackMiddleware\PageCache->handle(Symfony\Component\HttpFoundation\Request $request, $type = 1, $catch = true)
<ROOT>/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php:52 Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Symfony\Component\HttpFoundation\Request $request, $type = 1, $catch = true)
<ROOT>/vendor/stack/builder/src/Stack/StackedHttpKernel.php:23 Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Symfony\Component\HttpFoundation\Request $request, $type = 1, $catch = true)
<ROOT>/core/lib/Drupal/Core/DrupalKernel.php:716 Stack\StackedHttpKernel->handle(Symfony\Component\HttpFoundation\Request $request, $type = 1, $catch = true)

If you are editing an entity and you save the entity, you are redirected to the page showing the entity.
As in the entity content you added <img src="[node:my_custom_token]">, the browser is trying to access http://your-site-domain/[node:my_custom_token] to get the image data for the image it should show. Drupal doesn't find any route associated with that URL and it logs a Page not found error and throws an exception.

That isn't related to using tokens, as that could happen whether you enter a URL for your site that doesn't exist, for the src attribute.

If you are wondering why that token isn't replaced, that's because the entity form field isn't set to replace tokens, which it can also happens when the entity field is a Text (formatted, long) field for which none of the input filters set for its format handle tokens.