dovecot-optimize FTS Xapian Failed to open glass revision file for reading

Dears,

from time to time (no regular frequency) I get the following error:

/etc/cron.daily/dovecot-optimize: doveadm(account@domain.de): Error: FTS Xapian: Optimize (6) /var/vmail/mailboxes/domain.de/account/index/xapian-indexes/db_<longhex>/iamglass: Failed to open glass revision file for reading

The reason is the file "iamglass" is owned by "root", when I change this back to "vmail" the error disappears for a while. I do not find the cause why this sometimes changes. And it is only this file, where the ownership changes.

The content of /etc/cron.daily/dovecot-optimize is: #!/bin/sh doveadm fts optimize -A

I welcome any help / hint, thanks and kind regards Torge