zlib errors after upgrading

I get ZLIB Errors after dovecot upgrade from 2.3.10.1 to 2.3.11.3

Aug 21 15:27:34 lxc-imap dovecot: imap(acsida)<63870><jZk...>: Error: Mailbox Sent: UID=40826: read(zlib(/home/vmail/virtualmailboxes/acsida/storage/m.2409)) failed: read(/home/vmail/virtualmailboxes/acsida/storage/m.2409) failed: Broken pipe (FETCH BODY[])....

many of those entries....

after installing 2.3.10.1 those files are still broken - but new files are working again.

anything I can tweak or do ?