I've seen that same error message. What I do is go back to the login page and click Sign In a second time, and it takes me to the page I expect to see when clicking Sign In. I don't got a clue what's causing the glitch, but my little work around works and I can't complain.
BTW, I've noticed that I can also get to cPanel by closing the tab after seeing the error message and clicking the initial link (or bookmark in some cases) also gets me to the cPanel page.