From http://forums.mozillazine.org/viewtopic.php?f=39&t=2530463

Symptom:
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.

Cause:
The error happens with some servers that does not support “ID command”.

Solution:
Go [Option] – [Advanced], click [Config Editor] and fnd the following pref.

mail.server.default.send_client_info

It is true by default. Toggle it to false by double-clicking on it. Close the editor and restart Thunderbird.