zlib.read errors after upgrade from 2.3.15 to 2.3.17.1

[Arno Schneider]

After upgrading my mailserver from buster to bullseye and dovecot 2.3.15 
to 2.3.17.1 I got the following errors:

Jan 22 14:22:32 Warning: 
doveadm(user1)<2953060><49b0NRQF7GFkDy0ALVPUUA>: mdbox 
/local/mail/user1/storage: rebuilding indexes
Jan 22 14:22:39 Error: doveadm(user2)<2953112><awsPGB8F7GGYDy0ALVPUUA>: 
dsync(2nd.dovecot.server): 
read(compress(/local/mail/user2/storage/m.4650)) failed: 
read(/local/mail/user2/storage/m.4650) failed: 
zlib.read(/local/mail/user2/storage/m.4650): unexpected EOF at 7038367 
(last sent=mail, last recv=mail_request (EOL))

I have 2 dovecot servers in a multi-master configuration. The both 
servers are synced with dsync and the users are routed through a dovecot 
directors.

Any hints where I can continue to investigate / solve this issue?

best regards, arno