Dovecot 2.4.1 on DietPi/ARM64: Cannot move/delete any folders via IMAP, and three specific folders are unreadable (but other folders work)

Hello,

I'm running Dovecot 2.4.1 on a Raspberry Pi 4 with DietPi (Debian 13, ARM64) and had to migrate from another OS with Dovecot 2.3. My maildir is at /var/mail/mailuser. It contains many dot‑prefixed subfolders, e.g. .Archiv, .Archiv.Reisen, .Sozio. Relevant parts from doveconf -n:
text

dovecot_config_version = 2.4.0
auth_mechanisms = plain login
dovecot_storage_version = 2.4.0
listen = *
login_trusted_networks = 192.168.178.0/24
mail_driver = maildir
mail_home = /var/mail/%u
mail_path = /var/mail/%u
protocols = imap
ssl = no
passdb pam {
  driver = pam
}
userdb passwd {
  driver = passwd
}
namespace inbox {
  inbox = yes
  prefix = 
  separator = .
}

Problem 1: Cannot move, delete, or rename any folder via IMAP.
In Thunderbird, I can move individual messages between folders, but I cannot move, rename, or delete any folder itself. This affects all folders, not just specific ones. Right‑clicking a folder and choosing “Delete”, “Rename”, or dragging a folder to another location does nothing – no error, just no effect. I have checked permissions on the maildir; mailuser owns the directories and files, and dovecot is in the mailuser group.

Problem 2: Three specific folders cannot be opened (message body not loaded).
Most of my archive works fine. But exactly these three folders fail:

    .Archiv.merk.Versicherungen.Axa (Reiserücktritt)

    .Archiv.merk.Versicherungen.Bavaria_Haftpflicht

    .Archiv.merk.Versicherungen.LVM_Auslandsreisekrankenversicherung

In Thunderbird, I see the message headers, but when I try to open a message, the body stays empty. On the server, the files are intact – head shows the full header, and the messages are readable as plain files. Other folders with similar special characters (e.g. &APw-, &AOQ-, &ANY-) work without problems. So I don't think the special characters are the cause.

Problem 3: doveadm mailbox list shows only INBOX and Trash.
Even though the maildir contains many dot‑prefixed subfolders, doveadm mailbox list -u mailuser returns only:
text

Trash
INBOX

In Thunderbird, the subfolders appear inside INBOX (e.g. INBOX.Archiv, INBOX.Archiv.Reisen), which is basically correct. But doveadm does not list them as separate mailboxes.

What I have already tried (without success):

    Renaming the three problematic folders to simple names (e.g. Axa_Reiseruecktritt) and creating new folders with the copied mails.

    Deleting Dovecot index files (dovecot.index*, dovecot-uidlist, subscriptions) and restarting Dovecot.

    Changing namespace separator and prefix settings.

    Adding dovecot to mailuser group and adjusting permissions (chmod 750).

    Forcing Thunderbird to re‑synchronize by deleting local .msf files – after that, the three folders sometimes disappear or become empty in Thunderbird, but the mail files remain on disk.

None of this made doveadm mailbox list show the subfolders, and the three folders remain unreadable.

My questions:

    Why does doveadm mailbox list not show the subfolders, even though they exist as dot‑prefixed directories and Thunderbird displays them? What do I need to change so that Dovecot lists them properly?

    Why can I not move, delete, or rename any folder via IMAP, even though moving messages works? Is this a configuration issue (e.g. missing ACL, wrong permissions, or something with the namespace)?

    How can I make the three unreadable folders readable again? What is the safest way to rebuild their indexes or re‑create them so that Thunderbird can open the messages?

    If the folder names are problematic (e.g., due to length or special characters), what is the correct way to rename them in the maildir structure so that Dovecot and Thunderbird can handle them normally? I need a method that preserves the existing mails and the folder hierarchy.

I would be very grateful for any hints, because I'm stuck and have already spent many hours on this.

Thank you in advance.
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