Planned backup causes manually-started backup to hang

An unexpected (and annoying problem) happened to me today. My PC (64-bit Vista) has Acronis True Image 2014 which until now has not given me any trouble (and is better at cleaning up old backups than the previous version I owned).
My PC had at the time of the problem 3 USB disk drives attached.
I had started a (monthly) manual backup of my c: drive to one of the USB drives shortly before 10:00 am. This normally takes about 2 1/2 hours to complete. I was then away from the PC for over an hour.

A planned incremental backup (backing up from one of the other USB drives to the third one) was scheduled to start at 11:00, i.e. while the manual backup was still running. This incremental backup takes usually only a minute or two to complete.
When I returned to the PC at about 12:00, it was clear that the manual backup was stuck. It claimed it was 50 minutes from completion, but there was no activity on the USB target drive (and the 50 minutes displayed did not change).

After waiting for about 15 minutes, I decided to cancel the manual backup, but it did not appear to respond, so I had to stop Acronis using the Vista Task Manager.

When I restarted Acronis, the planned backup from 11:00 started, and ran from 13:01 to 13:02. After deleting the incomplete manual backup file from the morning, I then re-started the manual backup of my c: drive and it completed normally after 2 hrs and 25 mins.

It is clear to me that the manual and planned backups blocked each other in some sort of "deadly embrace." Is this a known problem? There was no overlap (as far as disk drives are concerned) between the two jobs.

PS When I try to look at the log it hangs