doveadm sync failed after adding "mechanisms = plain login" to passdb

Hello,

After adding mechanisms = plain login to passdb (passwd-file), doveadm -D sync -u e@example.com -d -N -l 30 -U failed with

Jan 26 06:49:22 doveadm(e@example.com): Error: 
remote(server2.example.com:61526): doveadm(1.1.1.1,e@example.com): 
auth-master: passdb lookup(e@example.com): Auth PASS lookup failed
Jan 26 06:49:22 doveadm(e@example.com): Error: 
remote(server2.example.com:61526): doveadm(1.1.1.1,e@example.com): 
/run/dovecot/auth-userdb: passdb lookup failed (to see if user is 
proxied, because doveadm_port is set)

With auth_debug=yes, I found these lines.

Jan 26 04:55:37 server2 dovecot: auth: Debug: skipping passdb: mechanism 
filtered
Jan 26 04:55:37 server2 dovecot: auth: Error: All password databases 
were skipped

What should I do to fix it without deleting mechanisms = plain login? More information could be found on this link.

Thanks