Acronis Backup plugin for WHM and cPanel: download or recovery of databases fails with "MySQLWaitingPidTimeoutError"

Symptoms

Download or recovery of databases fails. The following error message is shown in the operation log (if you click the failed task, and then View Details):

"Waiting existence \"{}\" file timeout exceeded".format(path) acronis_backup_srv.helpers.TimeoutException: Waiting existence "/usr/local/cpanel/base/3rdparty/acronisbackup/srv/tmp/dbms/9/mysql/mysqld.pid" file timeout exceeded

Raise MySQLWaitingPidTimeoutError() acronis_backup_srv.async_tasks.executors.databases.MySQLWaitingPidTimeoutError