If you have you tried the default IMAP or POP Generic option, automatic and manual methods of adding your iCloud account in Inky and you still get the following error:
"A problem occurred. The server rejected your username and/or password. Please start over and re-enter your email address and password."
Here are some suggested solutions:
1. Disable email scanning in your antivirus' settings and try adding the account to Inky again. Antivirus software like Avast, Kaspersky, etc interfere with Inky's operation .
2. If you are using aliases, please use the primary iCloud email account.
3. If you are using a two-factor auth, (common on El Capitan Os X), please turn this off and give Inky another try.