I copied the upgrade package for 5.0.1 over my existing 4.9.1 installation and triggered the install - all appeared to be fine - no errors thrown during install, site came back up just fine.  Tested registering new user, etc. all appeared to be okay, but now I cannot navigate to the "manage profile" page, even logged in an admin or host.  Checked the install log (at bottom of Host Settings page) and find these errors were logged but never reported:

Error: Cannot insert duplicate key row in object 'dbo.Packages' with unique index 'IX_Packages'.
The statement has been terminated.
Error: Could not find file 'D:\websites\<IP of my host>\<my domain.com>\main\admin\Portal\SiteSettings.ascx.vb'.
Error: Access to the path 'D:\websites\<IP of my host>\<my domain.com>\main\admin\Portal\sql.ascx' is denied.
Error: Could not find file 'D:\websites\<IP of my host>\<my domain.com>\main\admin\Security\roles.ascx'.
Error: Could not find file 'D:\websites\<IP of my host>\<my domain.com>\main\admin\Tabs\Tabs.ascx.vb'.
Error: Could not find file 'D:\websites\<IP of my host>\<my domain.com>\main\admin\Users\MemberServices.ascx'.
Error: Access to the path 'D:\websites\<IP of my host>\<my domain.com>\main\bin\DotNetNuke.DNNScheduler.dll' is denied.

None of the files noted above (SiteSettings.ascx.vb, sql.ascx, roles.ascx, Tabs.ascx.vb, MemberServices.ascx, DotNetNuke.DNNScheduler.dll) exist in the folders noted.  Anybody have an idea of what happened and what is the safe way to remedy?

Thanks in advance to all who take the time to investigate and reply!
 

After much noodling around with fresh install of 5.0.1 and comparing modules installed, database table contents, etc., I found that the User page setting for the portal in question had gotten set to the home page.  The really strange thing was that even if I went to the Admin->Pages list and picked the Manage Profile page, it would not display.  I could edit the settings for the page, but not view the page itself.  As soon as I tried to view the page, it redirected me to the home page, which I was taking to mean an error had triggered the default page, when it was actually just going to the page that was set as the User page for the portal. The User page setting can be accessed from the Host menu -> Portals -> edit the portal settings -> Advanced Settings -> Page Management, or from the Admin menu -> Site Settings -> Advanced Settings -> Page Management.  This was a very tricky issue to resolve, especially since the Security module that appears in the Module definitions list of a new install was not even listed in the site that I had upgraded. 

To get the security module to be available in my upgraded site, I had to create an install package zip file from the fresh install of 5.0.1, then install that packaged module into my upgraded site.  There were warnings that the module already existed in the site and I had to check the box to give it the okay to install over the existing module, but that process seems to have worked perfectly and I now have the security module available to put on any page.

This was not an intuitive process, but overall I think that the DNN guys have done a great job of providing tools to get things done with the environment, and I believe the support forum is very helpful whenever problems come up that require technical understanding of issues.  Thanks to all of you for your help!