Disclaimer: I’m not familiar with the backup/restore process.
Dovecot complains about wrong permissions in /var/mail.
Can you find a way to compare the content of this directory with its content from the original yunohost instance?
For comparison here is what I have on my cube (only two users, ‘toto’ and ‘tata’):
# ls -l /var/mail/
drwx--S--- 2 vmail mail 4096 juin 20 15:41 tata
drwx--S--- 10 vmail mail 4096 juin 20 11:17 toto
# ls -l /var/mail/toto
drwx--S--- 2 vmail mail 4096 juin 20 11:16 cur
-rw------- 1 vmail mail 396 juin 20 11:17 dovecot.index.log
-rw------- 1 vmail mail 120 juin 20 11:16 dovecot.mailbox.log
-rw------- 1 vmail mail 51 juin 20 11:17 dovecot-uidlist
-rw------- 1 vmail mail 8 juin 20 11:16 dovecot-uidvalidity
-r--r--r-- 1 vmail mail 0 juin 20 11:16 dovecot-uidvalidity.5b2a1b8a
-rw------- 1 vmail mail 7 juin 20 11:16 maildirsize
drwx--S--- 2 vmail mail 4096 juin 20 11:16 new
-rw------- 1 vmail mail 31 juin 20 11:16 subscriptions
drwx--S--- 2 vmail mail 4096 juin 20 11:16 tmp
Maybe the restore process did not set the correct owner/group names.
You may fix this manually:
sudo chown -R vmail:mail /var/mail/
PS: you can safely ignore all the warnings from slapd (mdb_equality_candidates… not indexed)