Thunderbird shows a popup that says, “The current operation on ‘Inbox(or one of your folders)’ did not succeed. The mail server for account (your account name) responded: Command Error. 12.”, when it tries to access an IMAP account.
The error happens with some servers that does not support “ID command”.
Go [Option] – [Advanced], click [Config Editor] and fnd the following pref.
It is true by default. Toggle it to false by double-clicking on it. Close the editor and restart Thunderbird.