Cannot update with 1.6 / Inactive Backup Now link / exec() has been disabled

I’ve just upgraded to WP 3.5 and plugin version 1.6. I made an automated update yesterday, before updating.

Now when I try to do a manual backup, the Backup Now (localhost) link is inactive.
It links to http://www.thesite.com/wp-admin/admin.php?page=Pressbackup_main&function=dashboard#
Nothing happens when I click it — no errors are reported.

However, when I go to the Compatibility tab, a box states that

exec() has been disabled for security reasons
In /thesite/public_html/wp-content/plugins/pressbackup/lib/Misc.php
On line 196

How do I solve this problem so I can backup the site again?

Kind regards,
and thank you for a up till now wonderful plugin!

PS. The same thing happened on a multisite I run, and I thought that was the problem. However, the page is not a multisite. I updated the multisite in December, and automated backups have stopped. There, I get the same error.

http://wordpress.org/extend/plugins/pressbackup/