My Angular application is getting random 504 errors when requesting various endpoints of my backend server (.NET).

Why random ?

• The same endpoint can be hit without errors from the same/different user at another moment during the day
• My backend server is serving successfully lots of requests at the same time other users get the 504 errors.
• Sometime my users have 504 errors and sometime they get CORS errors like Access to fetch at 'https://my-backend-server/endpoint' from origin 'https://my-frontend-app' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

What did I try ?

• .NET : Change CORS configuration to .AllowAnyOrigin().AllowAnyMethod().AllowAnyHeader()
• .NET : Add .SetIsOriginAllowed((host) => true) on my CORS policy
• Azure : Scale-up my App Services
• Angular : Add an RxJS timeout on my HttpInterceptor (the one I use to inject my JWToken) : return next.handle(request).pipe(timeout(this._defaultTimeout));
• Angular : Add ApplicationInsights both on my App Services to better troubleshoot the errors (now I exactly know that a lot of users get these errors everyday ????)
• Angular : Improve my <meta http-equiv="Content-Security-Policy" tag to include all external calls in every possible section
• Azure : Disable CORS on both my App Services (crashed the apps)

For information, I did not find any pattern on the failed requests (GET, POST), with or without authentication/authorization, during the day or low activity periods (~10pm), different controllers, etc.