Today I tried the new GUI Preview of TeamViewer 12 for Mac OS X. When I selected the Computers & Contacts panel, the GUI requested that I sign in. This request seemed odd, because when I had signed in previously using the old GUI for TeamViewer 11, it had displayed my Computers & Contacts panel with me already signed in.
I tried four times to sign in, each time re-checking that the e-mail address & password were correct. Four times I got the error message shown below:

So I changed the TeamViewer preferences to go back to the old GUI, closed TeamViewer, and then reopened it. Yep, as in the past under TeamViewer 11, the Computers & Contacts window was shown, without my having to sign in anew.
I reverted the preferences to opt again for the new GUI Preview, shut down TeamViewer, reopened TeamViewer, selected the Computers & Contacts panel, and -- lo and behold! -- that panel displayed without asking me to sign in anew.
After that, I was able to sign out and sign back in using the new GUI Preview
So now I am wondering if this was actually a transient malfunction of the sign-in server? Or was it due to my trying to run TeamViewer 12 for the first time with the new GUI Preview?
If you, too, had the sign-in server fail to respond to you, would you please reply to me?