I have this problem also, though not always with the flashing. I try to sign in to iTunes (12, Win 10 64) with my AppleID and password (as I have here) but the app throws "ID or password invalid" error.
I logged on, online, updated security, changed the password. Logged out and back on successfully to iCloud, A-OK. But the iTunes program still has the problem.
Uninstalled the program, then rebooted, then reinstalled. Same problem.
Cleared the app cache, shut down iTunes and restarted. Problem persists.
This appears in threads around the web, dating back to at least 2013.
Please, Apple; what is the solution?
I pray that it is not to switch to the Amazon system. Please!