There are a lot of tasks in "starting" status in Dropbox Extension

Symptoms

There are a lot of tasks in "starting" status in Dropbox Extension for a domain in Plesk > Domains > example.com > Dropbox Backup > Current Tasks. The tasks are in "starting" status forever and none of them ever starts.

The following errors appear in /var/log/plesk/panel.log with debug mode enabled:

CONFIG_TEXT: ERR [1] Zend_Db_Statement_Exception: SQLSTATE[HY000]: General error: 1 no such table: tasks
file: /usr/local/psa/admin/externals/Zend/Db/Statement/Pdo.php
line: 61
code: 0
DEBUG [extension/dropbox-backup] Dropbox Upload: /usr/local/psa/PMM/tmp/dropboxgphn3h.tar
ERR [1] Exception: PHP Warning: Error while sending QUERY packet. PID=667985; File: /usr/local/psa/admin/externals/Zend/Db/Statement/Pdo.php, Line: 228
file: /usr/local/psa/admin/plib/Smb/Exception/Syntax.php
line: 57
code: 0

Cause

Corrupted installation of Dropbox extension.

Resolution

1. Re-install Dropbox: Plesk > Extensions > Check Dropbox Backup > click Remove

2. Install it back: Plesk > Extensions > Extensions Catalog > Dropbox > click Install