Yes. I noticed only now that the maintenance task running anyway.
You are right, the error is probably caused by concurrency.
Thanks also for your suggest. I will setup a cron job.
I would also signal you that in the page /www/admin/maintenance-maintenance.php there are two broken hyperlink:
Anchor to "manutenzione programmata" in the text ( http://www.revive-adserver.com/docs/maintenance ) and the button Help in the up-right corner http://www.revive-adserver.com/docs/admin/settings/maintenance
Thanks a lot for your reply and for the best application that you build