Issue finally found. In our case the sync application message number was 33 which is responsible to trigger a sync and it was in status finished, while it must be in status waiting continuously. Attached SQL statements can force it into status Waiting, and if there would be errors can clear the error text field.

Checks the status

select * from fndcn_application_message_tab where application_message_id = '33';

Forces it in status Waiting:

update fndcn_application_message_tab set state = 'Waiting' where application_message_id = '33';

Clears the Error_text

update fndcn_application_message_tab set ERROR_TEXT = '' where application_message_id = '33';