Mailbox folder appears empty but contains files on disk

I decided to switch my custom mail server install (postifx, dovecot, SOGo) to mailcow (thanks a lot for this fine piece of software).
After installation, I set up the mailboxes, domains etc. Then, I copied over the Maildirs of the previous install (preserving timestamps, changing uid & gid) to the Docker volume.

The inbox looks fine (emails do not sort correctly in SOGo, though), but the sent messages folder (Sent Messages with a space) is empty (despite being 1 GB on disk). Opening the source folder in mutt confirms that all the mails are there. Similarly, I can open the the folder in the Docker volume (/var/lib/docker/volumes/mailcowdockerized_vmail-vol-1/_data/domain.tld/user/Maildir/Sent Messages) and, again, see all the mails.

The logs do not show anything, as far as I can tell. I already tried renaming the folder to Sent in order to avoid the space character, but this didn’t work either.

The /var/lib/docker/volumes/mailcowdockerized_vmail-vol-1/_data/domain.tld/user/Maildir/ looks like this:

drwx------ 23 5000 5000 4.0K Jan  2 02:58  .
drwx------  3 5000 5000 4.0K Jan  2 02:58  ..
drwx------  5 5000 5000 4.0K Jan  2 02:55  .Archive
drwx------  5 5000 5000 4.0K Jul  9  2018  Archive
drwx------  2 5000 5000 952K Jan  2 02:58  cur
drwx------  5 5000 5000 4.0K Jan  2 01:29 '.Deleted Messages'
-rw-------  1 5000 5000    0 Jan  2 01:21  dovecot-acl-list
-rw-------  1 5000 5000 231K Jan  1 12:02  dovecot.index
-rw-------  1 5000 5000  12M Jan  1 13:31  dovecot.index.cache
-rw-------  1 5000 5000  11K Jan  1 13:31  dovecot.index.log
-rw-------  1 5000 5000  33K Dec 30 13:37  dovecot.index.log.2
-rw-------  1 5000 5000   90 Jul  8  2018  dovecot-keywords
-rw-------  1 5000 5000   72 Jul 13  2018  dovecot.mailbox.log
-rw-------  1 5000 5000 664K Jan  2 02:58  dovecot-uidlist
-rw-------  1 5000 5000    8 Aug 23  2018  dovecot-uidvalidity
-rw-------  1 5000 5000    0 Jul  8  2018  dovecot-uidvalidity.4e60e599
drwx------  5 5000 5000 4.0K Dec 17 10:10  .Drafts
drwx------  5 5000 5000 4.0K Dec 15 13:29  Drafts
drwx------  5 5000 5000 4.0K Jul  9  2018 '.Entw&APw-rfe'
drwx------  5 5000 5000 4.0K Jan  2 01:29  .Junk
drwx------  5 5000 5000 4.0K Dec 22 22:26  Junk
drwx------  5 5000 5000 4.0K Jul  9  2018  .Junk-E-Mail
-rw-------  1 5000 5000  701 Jan  1 12:22  maildirsize
drwx------  2 5000 5000 4.0K Jan  2 02:58  new
drwx------  5 5000 5000 4.0K Jan  1 13:22  Notes
drwx------  5 5000 5000 4.0K Jan  2 02:55  .Papierkorb
drwx------  5 5000 5000 4.0K Aug 23  2018  Papierkorb
drwx------  5 5000 5000 4.0K Dec 17 10:10  'Sent Messages'
drwx------  5 5000 5000 4.0K Jan  2 01:29 '.Sent Messages'
drwx------  5 5000 5000 4.0K Jul  9  2018  .spam
-rw-------  1 5000 5000   86 Jul 13  2018  subscriptions
drwx------  2 5000 5000 4.0K Jan  2 02:58  tmp
drwx------  5 5000 5000 4.0K Jul  9  2018  .Trash
drwx------  5 5000 5000 4.0K Jan  1 12:22  Trash

How do I get this Sent Messages folder to work?