[horde] Duplicate entry in horde_activesync_state

[lg at wb7.eu]

Hello everyone,

I am using the current version (5.2.21) of Horde on PHP 7.1, Debian based.

Since today, syncing tasks over ActiveSync is suddenly impossible - neither Outlook 2016 nor Task&Notes on Android are able to sync. When trying to sync, the log file fills with entries like this:

2017-08-16T12:47:38+02:00 ERR: HORDE SQL QUERY FAILED: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '{59941a2f-314c-47a8-b8a1-7d0dbc442056}4' for key 'PRIMARY'
        INSERT INTO `horde_activesync_state` (`sync_key`, `sync_data`,
          `sync_devid`, `sync_mod`, `sync_folderid`, `sync_user`,
          `sync_pending`, `sync_timestamp`) VALUES (?, :binary0, ?, ?, ?, ?, ?,
          ?) [pid 18045 on line 282 of "/var/www/horde/htdocs/pear/php/Horde/Db/Adapter/Pdo/Base.php"]

Is there any idea how to fix this? I've tried removing the entries manually from the database, but they reappear (with a new UUID). I don't know why this happens since today.

Thanks in advance!

Regards,
Lucas