problems to mount - "image ist bereits gemountet"

I can't mount the backup, I get an error it is already mounted.

I did one full and one diferential backup of the "computer" (2 drives) on PC 1.
Backup_C_E_Ziel_G(2).tib
Backup_C_E_Ziel_G(2)2.tib
I mounted the second file on the PC 2 with write access to the mounted backup (the owner of some folders need to be changed) as S: and T:
a third incremental backup was created by Acronis with incremental changes
Backup_C_E_Ziel_G(2)3.tib
I did not unmount the backup before restarting the PC 2.

Now I can't mount any of this 3 files, I always get the error: "Das Image ist bereits gemountet".
So I started to browse the backup without mounting it and I tryed to copy data from the backup to the target PC. The copy process started but then it stopped without any message.

How I can mount the backup?
How I can use the backup?
How I can trust the backups created by Acronis when I can't use them when I need them?

I use Acronis true Image 2009 (build 9.809) on Windows 7 Premium Home 64 bit.

Germo