Internet Accounts preferences can't connect to the account …
Updating 17th October to Yosemite and working fine until 3rd November.
Mail was suddenly not able to connect to one of my external hosted e-mail accounts. Another account, same external virtual hosting but other domain, username, pwd is still working fine. Account settings in Mail are exact the same for both accounts (IMAP and SMTP). It's now a week trying to find the problem without any result. Very strange is, that Mail is sometimes suddenly able to connect the invalid account: Last week if I was away from the Mac for 20 minutes, coming back, the account was online. I was able to do some mails and cleanup my inbox. It ends if i reboot the Mac. In my network all Mavericks Macs, iPhones, iPads are connecting well this account.
I did a Mail setup just with the invalid account in a clean Guest User. No chance to connect the invalid account. Adding the other working one was just a thing of seconds.
I found that in System Preferences > Internet Accounts for the invalid account a pop-up asks for "Enter the password for …". Doing so it comes up with "Unable to verify account name or password" with the only chance to Cancel. I think there must be a starting point to my problem.
Is anyone knowing about this? Any help is very welcome.
iMac (27-inch Late 2009), OS X Yosemite (10.10)