Server 500 error on portal/stylesheets - DNN 9.6

I upgraded my DNN site from v.8.3 to 9.6 the latest version. My site is loading fine but the portal/stylesheets are not loading. Technically the cases are as below:

1. When the app pool is set as 'Classic', the stylesheet portal/default.css loads fine in the browser. However the site does not load anad times out without any clear error message

2. When the app pool is set as 'Integrated', the stlesheet throws server 500 error. but the website loads without the stylesheets.

I tried the below fixes:

1. Checked for enabling static content

2. In my web.config the static content looks as below:

     <staticContent>
   <clientCache cacheControlCustom="public" cacheControlMode="UseMaxAge" cacheControlMaxAge="365.00:00:00" />
    <remove fileExtension=".svg" />
    <mimeMap fileExtension=".svg" mimeType="image/svg+xml" />
    <remove fileExtension=".woff" />
    <mimeMap fileExtension=".woff" mimeType="application/x-woff" />
    <remove fileExtension=".woff2" />
    <mimeMap fileExtension=".woff2" mimeType="application/x-font-woff" />
   

Error log in portal/_default/logs folder:

[Thread:24][ERROR] DotNetNuke.Framework.Reflection - MAI.Components.FeatureController
System.Web.HttpException (0x80004005): Could not load type 'MAI.Components.FeatureController'.
   at System.Web.Compilation.BuildManager.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase)
   at DotNetNuke.Framework.Reflection.CreateType(String TypeName, String CacheKey, Boolean UseCache, Boolean IgnoreErrors)

So the basic issue is with 'Integrated' app pool the portal/stylesheets does not load and produce server 500 error. Any clue on how this can be fixed?

Your app pool should be of the Integrated type. Classic is no longer supported.

You appear to be using a custom module with MAI as part of the name.

You should be aware that the upgrade from DNN 8.x to 9.6 (the latest version is 9.6.2, but that's not an issue) crosses the magic line at which several hundred deprecated methods were removed from DNN so many modules require adjusting and recompiling to work.

You should also check and verify that you are using the right version of the ASP.NET framework. I think that you need at least 4.7.2.

To summarize, I think that the problem is either with you module or with the version of ASP.NET, provided that your app pool is in Integrated Managed Pipeline Mode.