URL Is Not Valid and Cannot Be Loaded Error
I recently had this error pop up as I tried to log into my Drupal site from the main page (the error did not occur when I logged in from /user).
This error pops up when Drupal cannot resolve the path for the front page in the login form. The problem in my case was that I had the front page link as "/" (under settings -> general -> default front page). The problem was the backslash, which you are advised not do use, but I used it because I didn't like having the "node/3" attached to the URL when "forward this page" was selected. In my case the new link would show as //, instead of /node/3, and since I have .htaccess setup to remove trailing slashes (not generally recommended, but there's a specific reason I do this that I'll explain if you're interested - just comment below) it would resolve to /, which is the appropriate URI formulation.
I had to remove the trailing slash and now the login works fine. The form code now shows:
Hope this helps. If this did not solve your problem please comment below.