Generally the cause of Server Application Unavailable is the .net framework.
Error Snapshot
For ex. one of our site is in version 1.1 and the recently added site is in 2.0 or later or any other different versions running on the IIS.
How to Resolve:-
We need to create a new application pool on the IIS and assign the same to the version of the site.
Following image illustrates how to check the version of site.
For ex. Pool3_5 for version 3.5 and Pool4_0 version 4.0 .
Following image illustrates how to create new Pool.
Step 1:
Step 2:
Step 3:
You can also verify the IIS logs to locate the issue.
Error Snapshot
For ex. one of our site is in version 1.1 and the recently added site is in 2.0 or later or any other different versions running on the IIS.
How to Resolve:-
We need to create a new application pool on the IIS and assign the same to the version of the site.
Following image illustrates how to check the version of site.
For ex. Pool3_5 for version 3.5 and Pool4_0 version 4.0 .
Following image illustrates how to create new Pool.
Step 1:
Step 2:
Step 3:
You can also verify the IIS logs to locate the issue.