Drivemap changes lead to vicious circle

I've got the latest 2015 update (an hour ago) on Win7 and changed my backup drive from D: to E: (same drive, just mapped to a different drive letter) and I've got an incremental backup scheme.

I've changed the backup profile to back up to E: instead of D: but when I tried to create today's backup, it failed with a message "The specified file path does not exist Click retry to try to read from the same localtion. To create a new version according to your current backup scheme press Cancel"

So - retry simply regenerates the same message (as expected) so I try Cancel. This produces a message "Cannot access the path D:\...." and a nasty red X.

Pressing "Backup Now" again just goes around the circle.

I've tried "net use D: resourcenamefore:" which was successful and restarted Acronis, but to no avail.

How do I persuade this application either to read from E:, or accept the drive-map for D: or start a new cycle with a full backup?