Error at start of backup over Win 7 LAN

Build number is: 6525. License is for 3 PCs.
Acronis True Image 2015, running on Windows 7, Home Premium 64 bit. Under the conditions below, always gives error:
"Please close the app that may currently Use {path to backup *.tib file}." "To specify another location, click browse."

This error is while backing up over a Win 7 peer-to-peer network of 2 PCs, to a USB drive on the other Win 7 64 bit PC. The error happens about 10 minutes into the backup. There are no apps/programs accessing or viewing the USB backup drive when the error occurs. Nor is Windows Explorer running. This backup used to work, but fails now. Perhaps as the result of a recent Acronis T.I. update?

Acronis T.I. 2015 is installed and running on both PCs. On the remote PC, it works perfectly to a USB drive on that PC (not over the network). Both backups are scheduled to run at the same time in the evening, when the computer users have left work.

What I have tried so far:
*Restart both PCs - no fix.
What I have not tried yet:
*Uninstall Acronis T.I. and re-install it.
Any ideas?