Did this just start? 

Is this after an upgrade?

Have you tried restarting IIS or rebooting your server?

I just upgraded to DNN 4.9.0 and started getting this error as well.  My site is hosted by a service provider so it is not possible to simply reboot IIS. I have tried editing the web.config which causes IIS to restart the application but still get the error.  Anyone know how to force DNN to restart completely, or what is causing this problem?

The essential message is:

D:\Domains\katangapages.cd\wwwroot\admin\Host\FriendlyUrls.ascx.vb(44):
error BC30560: 'RewriterRuleCollection' is ambiguous in the namespace 'DotNetNuke.HttpModules.Config'

You mentioned that you upgraded to 4.9.0. From which version of DotNetNuke did you upgrade? It's possible that you have some extraneous code in your installation. We'll need to figure out which ones they are to fix this.

Hi

I just did an upgrade on my server from v4.8 to v4.9

I am getting this error
======================================
Server Error in '/' Application.
The resource cannot be found.
Description: HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable. Please review the following URL and make sure that it is spelled correctly.

Requested Url: /Host/Host Settings/tabid/16/portalid/0/Default.aspx

======================================

When I try to navigate to the Host >Host Settings page

I would appreciate any help you can give me.

Interestingly enough, I can access the Host>Host Settings page for another Portal, just not this PortalID 0?

ie.
Host/Host Settings/tabid/16/portalid/2/Default.aspx

If you have access to the database, check the Tabs table, and see if the host pages for PortaID=0 are marked as protected by having IsSecure=True.  Change those to False for Portal 0.

I seem to recall that this happens if you had the allow SSL checked in the site's admin setting when you did the update.

Ok, interestingly enough I got an SSL Certificate error after the update, whereby I had to add an exception to my browser for the site? Relevant?

I changed the IsSecure to false and uploaded the Web.config file again (To restart the App) and it started working.
Now it all seems to be working.

Note: The Host Tabs have NULL as the PortalID. I made the mistake of changing this, and had to change it back, as the Host Menu disappeared from one of my other Portals.

I think that this is an upgrade issue, and resulted from having "SSL Enabled" check in the Admin, Site Settings page.  At least, this was why it happened for me!

In the Tabs table, you should have found some host tabs with NULL for PortalID, but you should also have found that same collection with the PortalID for each of your portals.

As you found, it's the tabs with a non-NULL PortalID that you needed to change.