mail_daemon doesn't fetch mail from 3rd configured account

[humdinger]

This is hrev57208, 64bit.

This might be similar to #18028.

I used to have two GMail accounts (IMAP) that work perfectly. Now I added a third account at Mailbox.org (IMAP). The two GMail accounts are still operating OK, the Mailbox.org account isn't.
When I try fetching mail from just the Mailbox.org account, the mail_daemon output is short enough to paste here:

IMAP: sync
IMAP: worker 0x109d3c0e7f90: enqueue check subscribed folders
Connect
Login
C: A0000009 LOGIN "mynickname@mailbox.org" "MySuperSecretPassword"
* OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE UIDPLUS LITERAL+ AUTH=PLAIN AUTH=LOGIN] Dovecot ready.
CAPABILITY: IMAP4rev1, SASL-IR, LOGIN-REFERRALS, ID, ENABLE, IDLE, UIDPLUS, LITERAL+, AUTH=PLAIN, AUTH=LOGIN
A0000009 OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE SEARCH=X-MIMEPART XDOVECOT ACL UIDPLUS LITERAL+ NOTIFY SPECIAL-USE FILTER=SIEVE COMPRESS=DEFLATE XAPPLEPUSHSERVICE QUOTA ACL RIGHTS=texk] Logged in
capabilities: IMAP4rev1, SASL-IR, LOGIN-REFERRALS, ID, ENABLE, IDLE, UIDPLUS, LITERAL+, AUTH=PLAIN, AUTH=LOGIN
C: A0000010 ID NIL
* ID ("name" "Dovecot")
Server:
  name: Dovecot
A0000010 OK ID completed (0.001 + 0.000 secs).
C: A0000011 LSUB "" "*"
* LSUB () "/" INBOX
* LSUB () "/" INBOX/J.Seemer
* LSUB () "/" INBOX/Mailinglists
* LSUB (\Sent) "/" Sent
* LSUB (\Drafts) "/" Drafts
* LSUB (\Trash) "/" Trash
* LSUB (\Junk) "/" Junk
A0000011 OK Lsub completed (0.001 + 0.000 secs).
C: A0000012 LOGOUT
* BYE Logging out
Unhandled S: BYE, Logging, out
A0000012 OK Logout completed (0.001 + 0.000 secs).

No mails were fetched.

When I remove the two GMail accounts, so there's just the one Mailbox.org account, everything works perfectly. Attached is the log from that (those mails are mainly my new mailing list subscriptions).

[humdinger]

Mailbox.org only account works