If you are missing the midb.sqlite3 file, you have probably not have a well-created store.
here you can find the mapping of users to directories (stores):
echo "select username,maildir from users where maildir<>'';" | mysql -N grommunio
check if the user store is really completely created with the following command:
ls -l <maildir
Here you should see something like this:
drwxrwx--- 1 gromox gromox 103568 Jun 7 09:18 cid
drwxrwx--- 1 gromox gromox 126 Mar 1 14:51 config
drwxrwx--- 1 gromox gromox 10 Dec 23 2020 disk
drwxrwx--- 1 gromox gromox 54880 Jun 7 09:18 eml
drwxrwx--- 1 gromox gromox 82 Jun 7 09:18 exmdb
drwxrwx--- 1 gromox gromox 50894 Jun 7 09:18 ext
drwxrwx--- 1 gromox gromox 42 Dec 23 2020 tmp
and
ls -l <maildir>/exmdb
Here you should see something like the following:
-rw-rw---- 1 gromox gromox 9588736 Jun 7 09:18 exchange.sqlite3
-rw-rw---- 1 gromox gromox 77824 Dec 23 2020 midb.sqlite3
if not, you need to create the store forcibly (be careful, existing data is deleted!):
gromox-mkprivate -f <username>
gromox-mkmidb -f <username>
Please note that if you have already migrated a store (possibly partially incomplete), you should clean it up accordingly beforehand, i.e. if necessary (with caution!) an rm -f in the store directory and then create the stores again (see above).