Maybe you have the same issue http://world.episerver.com/forum/developer-forum/Developer-to-developer/Thread-Container/2016/8/login-page-not-found-in-cms-9/

If there is some virtual directory setup in iis as Apps, try by removing that

Btw, it's actually a bit unclear if you really end up on the 404 page or the login page. The URL looks like a login page, or do you mean that the URL you posted is in the querystring on the 404 page?

You say that you are migrating, what are you migrating from and to?

It could be that you are missing files in the folder /modules/_protected/

Or it might be that you are still pointing out the wrong path to this in episerverframwork.config (or accually the VPP-settings for epi)

Thanks for the responses. We copied the solution from one of our servers to another to do some stress testing. We moved the site and related database from production to a testing server.  It works just fine on production.

After we copied it (and the database) to our testing environment, the home page site was blank.  So I manually edited two records in tblBigTable to make the site recognizable both on localhost and on the testing server.  So the end-user facing side would render correctly.  But the admin section displays a 404 page, but has this in the URL:

mysite/Util/login.aspx?ReturnUrl=%2fepiserver

My initial thought was that perhaps there is another record in the database that needs to be adjusted to make the Admin section display properly.  The missing zip files mentioned in this post do appear in our copy.

http://world.episerver.com/forum/developer-forum/Developer-to-developer/Thread-Container/2016/8/login-page-not-found-in-cms-9/

Thanks so much for helping me out.