dovecot mailserver: Authentication server didn't send valid SPID as expected-Collection of common programming errors

I have set up my mail server and been using thunderbird to check my mails (followed the instructions here). Everything has been working well, except after I moved a few days ago, I can no longer check my mails using thunderbird. I check the logs on the server side and I see this error messages:

Apr  2 09:01:44 hostname dovecot: imap: Error: Authentication server didn't send valid SPID as expected: MECH^IPL
AIN^Iplaintext
Apr  2 09:01:44 hostname dovecot: imap: Error: Disconnected from auth server, aborting (client-pid=16739 client-i
d=1)
Apr  2 09:01:44 hostname dovecot: auth: Error: BUG: Authentication client sent unknown handshake command: REQUEST
?1089470465?16739?1?7a504e346ca8ecf52b8708ea8a3e5dd2
Apr  2 09:01:45 hostname dovecot: auth: Error: BUG: Authentication client sent unknown handshake command: REQUEST
?3807248385?16744?1?a3c7ad0ff58878582049c9db2ed88609

I suspected it might be due to change of my IP address. But I dont think I specify the range of IP that could access the server in dovecot. I only set that for Postfix and that has been fixed, yet the issue still stays.

Got any ideas? Thanks.