Here are some errors due to IIS, we come accross while working with .net application
Service Unavailable

We need to check the System Event Log for this error. There we can get different error or warning messages. The below points provide some of the error messages with some more specific problem. To go to Event viewer Start à Run à “eventvwr”, from there select the system
The identity of application pool ‘%1’ is invalid, so the World Wide Web Publishing Service can not create a worker process to serve the application pool. Therefore, the application pool has been disabled.

The application pool identity must be set to a valid account for the application pool to function.
A user is invalid if it is not a member of Internet Information Services Worker Process Group or has not been granted the right to login as a service. To add a user we can follow the below steps.

1. Start à Run à “secpol.msc” Enter
Expand the Local Policies node from the security settings then select User Rights Assignment.

2. Double click on Log on as Service or right click on it then go to properties.

3. Click on Add User Group and enter the name of the user for the application pools identity. We can find the user by clicking the Find now from the advance button.

4. Then we need a reboot to read the security policy.
Mutex could not be created.
This occurs on asp.net sites when we try to change the application pool while the IIS is running. To solve the issue just reset the IIS by Run àiisreset in the command prompt. If still the problem is there then go to the temporary folder for v2.o framework and remove the folder for our application. The path for temporary folder is
<Windows dir>\Microsoft.Net\Framework\v2.0<extra numbers>\Temporary ASPNET. While removing the temp folder please ensure the visual studio window is closed.
Parameter is incorrect.
This error occurs within IIS while setting up a new website with the IIS manager because our website has not been assigned a valid application pool or the default app pool has been deleted. If that is the case, then select an existing app pool in the website properties
No primary driver specified in configuration
Asp.net application is failing to find the web.config file.

HTTP 404: The page cannot be found
This error occurs when the IIS is unrecognized or prohibited. This is the case while both ASP.Net 1.1 and 2.0. We can follow the below procedures to fix the issue.

1. Open the “Internet Information Services (IIS) Manager” under “Administrative Tools”.

2. Navigate to the “Web Service Extensions” node.

3. Select “ASP.NET v1.1.4322” or “ASP.NET v2.0.50727” in the right pane.

4. Click “Allow”.