USB Backup Job Options sometimes disabled

I use USB Backup plugin and for all jobs I set archive mode -a what implies -rlptgoD and addistional I use "Preserve ACL". Just wanted to edit some of the jobs and there is only one Job where the options Preserve Permissions, Preserve Owner, .. Group and ACL are deactivated and locked. I assume this has sth to do with the fact that archive mode already implies some of them. But it happens only to one of my jobs and even deactivating archive mode does not let me activate the others again. Whats happening here? And why is ACL preservation affected in general?

Edit: Now it happened to all the other jobs too. I am simply not able to touch the buttons They are not active. Nothing helps but creating a new job.

Did some further investigation. The behavior occurs if I try to edit a job while the USB drive is not connected. The marked options are disabled and not changeable then. After connecting the USB drive again, I can finally edit it. Bug or supposed to do so?