I tried this one and it worked for me:

1. Go to 'Debug Properties'

2. Find 'Web Server Settings'

3. Change the port in 'App URL' section and save the changes

4. Run the application and the same error will appear again

5. Switch the port back to the original port and save the changes

6. Run the application and enjoy it!

I hope this trick work for you.

1- Go to 'Debug Properties', Find 'Web Server Settings', Change the port in 'App URL' section and save the changes

In your project folder, delete .vs file, also remove the launch settings file in properties (make sure you copy to a different folder before removing it).

Restart visual studio and your machine if possible. Go to launch setting, check if they are properly configured similar to your prev launch settings. Run the application in IIS.

Should start working.

The following fixed the problem for me.

• Delete the "properties\launchsettings.json" file. The above solution has worked for other users too as per https://developercommunity.visualstudio.com/t/unable-to-connect-to-web-server-iis-express-in-vs/1027196

From my experience this issue appears when the port used is already in use by other process. You will see the following:
Error description: The process cannot access the file because it is being used by another process. (0x80070020)
Just choose a port number which is free to use and the problem will disappear!

To me it has worked by switching the project from Debug to Release and Starting it. When I switch back to Debug it starts correctly. It appears to me that For some reason i had the Release project running so the Debug couldn't start.

I have just an API project. Going to project's Properties\Debug and unchecking the 'Launch browser' helped.

I encountered the same issue , but after few seconds i find out that there is a duplicated sslport , the same sslport being registered for another running app , so when i run another app with the same setting i see this error , you have to change the sslport in the launchsetting.json

In my case... (Visual Studio 2019, .net 5, blazor wasm project):

• I did bkp copy of "Properties\launchSettings.json" file and ".vs" folder (just in case)
• I deleted the "Properties\launchSettings.json" file and open the solution.. This file is recreated with default values...
• After that, the error is gone and I was able to run and debug the application normally.

Now you can redo your updates on the "Properties\launchSettings.json" (i.e. change port).