Cannot sync mailbox due to "SQLSTATE[22007]: Invalid datetime format"

Expected behavior

sudo -u www-data php occ mail:account:sync <account-id> should succeed.

Actual behavior

CLI-command throws an exception and account sync fails. If mail app is visited in the browser, the inbox never finishes loading.
CLI-log follows (some info redacted):

root@xxxxx xxxxx# sudo -u www-data php occ mail:account:sync 1
[debug] account is up to date, skipping mailbox sync
[debug] Skipping mailbox sync for Archive
[debug] Skipping mailbox sync for Aurox
[debug] Skipping mailbox sync for Drafts
[debug] Initial sync 1:INBOX - fetch all messages from IMAP took 1s

In ImapToDbSynchronizer.php line 235:

  Sync failed for 1:INBOX: An exception occurred while executing 'INSERT INTO `oc_mail_messages` (`uid`, `message_id`, `references`, `in_reply_to`, `thread_root_id`, `mailbox_id`, `subject`, `sent_at`, `flag_answered`, `flag_deleted`,
   `flag_draft`, `flag_flagged`, `flag_seen`, `flag_forwarded`, `flag_junk`, `flag_notjunk`, `flag_important`) VALUES(?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)' with params [4644, "<E1eSNge-0004vA-Pf@www.neuhold-elektronik.at
  >", "[]", null, "<E1eSNge-0004vA-Pf@www.neuhold-elektronik.at>", 7, "\x44\x69\x67\x69\x74\x61\x6c\x20\x4d\x75\x6c\x74\x69\x6d\x65\x74\x65\x72\x20\x34\x30\x30\x30\x20\x43\x6f\x75\x6e\x74\x73\x20\x73\x74\x61\x74\x74\x20\x80\x20\x33\x3
  9\x2c\x39\x35\x20\x6e\x75\x72\x20\x80\x20\x32\x39\x2c\x39\x35", 1513950517, false, false, false, false, true, false, false, false, false]:

  SQLSTATE[22007]: Invalid datetime format: 1366 Incorrect string value: '\x80 39,9...' for column `nextcloud`.`oc_mail_messages`.`subject` at row 1


In AbstractMySQLDriver.php line 106:

  An exception occurred while executing 'INSERT INTO `oc_mail_messages` (`uid`, `message_id`, `references`, `in_reply_to`, `thread_root_id`, `mailbox_id`, `subject`, `sent_at`, `flag_answered`, `flag_deleted`, `flag_draft`, `flag_flag
  ged`, `flag_seen`, `flag_forwarded`, `flag_junk`, `flag_notjunk`, `flag_important`) VALUES(?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)' with params [4644, "<E1eSNge-0004vA-Pf@www.neuhold-elektronik.at>", "[]", null, "<E1eSNge
  -0004vA-Pf@www.neuhold-elektronik.at>", 7, "\x44\x69\x67\x69\x74\x61\x6c\x20\x4d\x75\x6c\x74\x69\x6d\x65\x74\x65\x72\x20\x34\x30\x30\x30\x20\x43\x6f\x75\x6e\x74\x73\x20\x73\x74\x61\x74\x74\x20\x80\x20\x33\x39\x2c\x39\x35\x20\x6e\x75
  \x72\x20\x80\x20\x32\x39\x2c\x39\x35", 1513950517, false, false, false, false, true, false, false, false, false]:

  SQLSTATE[22007]: Invalid datetime format: 1366 Incorrect string value: '\x80 39,9...' for column `nextcloud`.`oc_mail_messages`.`subject` at row 1


In PDOStatement.php line 129:

  SQLSTATE[22007]: Invalid datetime format: 1366 Incorrect string value: '\x80 39,9...' for column `nextcloud`.`oc_mail_messages`.`subject` at row 1


In PDOStatement.php line 127:

  SQLSTATE[22007]: Invalid datetime format: 1366 Incorrect string value: '\x80 39,9...' for column `nextcloud`.`oc_mail_messages`.`subject` at row 1


mail:account:sync [-f|--force] [--] <account-id>

Mail app

Mail app version: 1.7.2

Mailserver or service: Exim 4.92 (from Debian)

Server configuration

Operating system: Debian 10.7

Web server: Apache/2.4.38 (from Debian)

Database: MariaDB 10.3.27 (from Debian)

PHP version: PHP 7.3.19 (from Debian)

Nextcloud Version: 20.0.4

Client configuration

Browser: Firefox 84.0.1

Operating system: Windows 10 20H2 (on browser), Debian 10.7 (on CLI)

Start with ImapToDbSynchronizer.php around line 235 and the reported INSERT failure in AbstractMySQLDriver.php. Reproduce sudo -u www-data php occ mail:account:sync <account-id> using the affected mailbox and inspect how the subject containing euro symbols reaches MariaDB. Done means the account sync completes and the inbox loads without the invalid-string error.