This happened about a year ago but I managed to fix by swapping the SSD for a backup one. I use Thunderbird or EmClient on win10 desktop. I have tried 3 ssd drives and an old hdd one. They all give the same message asking for a password which fails to work even though the password was saved. Tried other passwords to no avail. Laptop using Win7 gives the same as does my phone. This is the second day, could it be their server. I am using TalkTalk.