dovecot - Oct 2019 - Error: Mailbox INBOX: Broken file dovecot-uidlist: Invalid data

Hi list,

today a user reported messages are disappearing from his INBOX and I discovered
a two digit number of mailboxes with broken dovecot-uidlist files.

The log says:

Oct 29 08:51:34 mail02 dovecot: lmtp(info at
example.com)<5270><QOoVJIbvt12WFAAAJOVBUw>: Error: Mailbox INBOX:
Broken file /var/vmail/example.com/info/dovecot-uidlist line 88: Invalid data:

And the respective files do look broken:

4596 :1572344253.M529133P14861.mail03.example.com,S=53053,W=54233
^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@4597
:1572344797.M93833P15905.mail03.example.com,S=126866,W=129256

Setup : Behind a director I am running two dovecot backend servers sharing the
vmail directory via NFS.

Do you have an idea where this might come from? How can I debug this further?

Cheers,
Dirk