I don't understand what you mean by that Vitaliy - reverse incremental (you can tell it's that mode by the .vrb file) always rolls back changes to the full backup file after a failure. "Rolling back changes made by incomplete job run" is what you will see in the real-time statistics during this process. Perhaps you were thinking of forward incremental?
Pavel's right about the possibility of antivirus or similar conflict, but I also occasionally see a NAS decide to lock a file for no obvious reason and not let go until the NAS is fully powered off and back on again (I've even seen a soft reboot not fix it).
What is your support case number?