Exchange Copy to Tape: 1032 The file cannot be accessed because the file is locked or in use

We use BackupExec 2012 Version 14.0.1798 (64bit) on Windows Server 2008R2.

Problem: Exchange-Backup (GRT): B2D always works fine. Copy to Tape is planned some hours after. Job writes data without error but verificacion ends with: "1032 The file cannot be accessed because the file is locked or in use". On the tape we find all data (about 400 GB).

Questions: What file cannot be accessed?