500.121 errors can point towards the HTTP queue being backlogged with more requests than it can respond to within 232 seconds, which is the timeout limit. This is typically observed when CPU and/or memory usage is high on your web app. If one web app within an app service plan is resource heavy, it can impact other apps in the same app service plan since they share the same resources.
To resolve this, there are a few options you have.
If you have many web apps in an app service plan, consider putting the more resource intensive apps in their own app service plan.
Setting up auto scaling can also help resolve these matters. To learn more about configuring auto scaling, please see here.