The non-blocking error is:

This error occurs every time on my test machine (no traffic, only a few test messages). That error sometime also occurs on my production system, but not each time. 

The command I used for the first time is : doveadm backup -A -s "" -d > state.txt

After that there is a daily calls (as vmail user, my config only handle virtual users) to : 

doveadm backup -A -s "$(<state.txt)" -d > state.txt.tmp && mv state.txt.tmp state.txt

doveconf -n output is available at https://www.f-hamelin.fr/pub/doveadm-backup-doveconf.txt

Any help will be welcomed 

Best regards,

Franck