I better dig deep into this issue. Just upgraded two big sites. One community and one Professional Edition and find this issue on an important module that clients use all the time,.
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An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. Stack Trace: [NullReferenceException: Object reference not set to an instance of an object.] DotNetNuke.Entities.Portals.PortalSettings..ctor(Int32 tabID, PortalAliasInfo objPortalAliasInfo) +49 DotNetNuke.HttpModules.UrlRewriteModule.OnBeginRequest(Object s, EventArgs e) +2087 System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +68 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

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1. Is site's application pool running under ASP.Net 4.0 ?
Yes, and in integrated mode

2. What is the web server's IIS version?
IIS7

3. Are the website's bindings using a non-standard Http port (i.e. not default 80)? If so, is the UsePortNumber node in the appSettings section of web.config a) commented out, b) Set to True or c) Set to False?
Default port. UsePortNumber node was commented out.

4. Are some or all png image files in the site's skin or in use elsewhere on the site showing missing image icons or errors?
No issue with PNG files

5. If you can log-in as host user, please check the Admin-->Site Settings Portal Alias listing or look at the PortalAlias table in the database to see if it contains any new entries - all containing references to png image files. If so, do these entries reappear after they are manually deleted and the site is again visited?
Not getting this PNG issue on mine - only the failure to view the portals with the root domains.

6. Do you recall how you first hit the site to trigger the upgrade or install - a) Site url without "/default.aspx" at end, b) Site url with "/default.aspx" at end, c) Site url with "/Install/Install.aspx?mode=upgrade" or "/Install/Install.aspx?mode=install" at end?
Site url with "/Install/Install.aspx?mode=upgrade

7. In the sites IIS configuration, are any image file extensions (particularly .png) mapped through the ASP.Net (ISAPI) handlers?
Nope, no image file extensions there at all

My system has been upgraded since DNN 4.3.4 and has been running 5.5.1 until I attempted this upgrade.

I tried a second round with the UsePortNumber node active, but I ran into some sort of redirect loop and had to give up and restore again. Leaving it there.
Rob

Hi,

I activated url rewrite version 2.2.0 of Ifinity and no event errors in IIS application log anymore. No .ico, .png or other portal alias error anymore. During upgrade I got next errors of which the first happens since the moduletitle column has been removed manually during failed upgrade to dnn 5.5.0/1 and reverting back to dnn 5.1.4.

Other errors resolved by installing the telerik manually (and adding to bin the dll's since otherwise not working). the cachingproviders also uploaded manually through the module host. Also I copy-pasted with exception of the machine key and sql connection a working web.config to the beta since upgrading failed on production server (but worked on dev).

00:00:17.734 -   Executing Application Upgrades: 05.03.00 Error!
00:00:20.015 -   Executing Application Upgrades: 05.06.00 Error!
00:00:20.234 -   Cleaning Up Files: 05.06.00 Error!
00:00:21.437 -   Installing Package File Telerik_05.06.00_Install:  Error!
00:00:26.625 -   Installing Package File FileModuleCachingProvider_05.06.00_Install:  Error!
00:00:26.718 -   Installing Package File MemoryModuleCachingProvider_05.06.00_Install:  Error!

The question now is: is DNN 5.6.0 with Ifinity stable on asp.net 4.0 for more than 1 user?

J.