I could not post the solution to the problem posted under the same Subject earlier. To save others time, that run into the same issue, I repost Q und the answer. :-)
(Q) I have upgraded recently to 6.1.4. without any issues. Now, upgrading to 6.1.5, I suddenly have a problem with the same upgrade process. When I enter my super-user credentials and press "Update Now" I receive a "page not found" error.
(A) I fixed the issue. There was a URL rewrite rule set in IIS7 that worked fine on the DNN portal(s), but not on the DNN upgrade procedure. I stopped the rewrite rule, did the upgrade, switched the rule back on, done.