A failure was encountered while launching the process serving application pool

Have a look of there any warning before this eror in the vent log, it sometimes throws a warning with a better description of the problem and then the error.

If you are still having the problem, try to add your app pool account into the local administrators group.  I am not sure if it is needed, but it works for me most of the times.

The first thing I would do is go to the Application Pool properties, and re-enter the credentials for the Identity account.

Sometimes the stored password gets corrupted.  Then, after a failed attempt or two at authenticating itself, it disables the Application Pool.

HTH...

I got this error.  this is what worked for me:

Application Pool Rights

The K2 Workspace Service Account will require elevated permissions to run the application pool.  Use the aspnet_regiis command to configure this.  This tool ships with the .NET Framework, and takes the pain out of configuring all the required NTFS permissions, IIS_WPG group membership, security policy user rights assignments, and IIS metabase access rights.  For more information, see the MSDN article on setting security rights for .NET Applications, at http://msdn2.microsoft.com/en-us/library/ms998297.aspx.

To use the aspnet_regiis command, perform the following steps:

1. 
   
2. Open a command prompt (Start > Run > cmd)
   
   
3. Change directories to the .NET Framework folder (C:WINDOWSMicrosoft.NETFrameworkv2.0.50727)
   
   
4. Type aspnet_regiis -ga domainK2 Workspace Service Account and hit Enter
   
   
5. After the command completes, type iisreset and hit Enter

If you are running the 64 bit version you will need to change the path to:

C:WINDOWSMicrosoft.NETFramework64v2.0.50727