Tape based backup failure - Capacity Issue?

I have been running a tape based backup of my file server for some time now, and up until recently, there had been no issues.
I am using the following versions on a MS Server 2003 box:
Acronis Backup and Recovery 11 Server for Windows
Acronis Backup and Recovery 11 Agent (build 11.0.17318)

I take a full backup on a nightly basis to an Ultrium LTO3 tape drive 400\800GB capacity.
Recently, the job has started failing at a certain point each night. The progress of the task reports at 99%, at which point the following entries are logged, and the tape is ejected. The successful jobs show around 450GB of data is being backed up.

I have the following settings configured on the jobs:

Tapes are in Free Pool.

Compression Level is set to Normal

Additional settings include:
Overwrite new tape when performing full backup: On
Always use free tape (have tried with this function not selected): On
Ask for the first media while backing up to removable media: Off

Eject Tape when full is not selected.

Can anyone assist as to why the tape is being ejected and not starting until a new tape is inserted. I would assume it is a capacity issue, but I would expect more than 450GB of data to be stored on a LTO3 tape (compression ratio shows as 1:1.1 for previous jobs). And the 'Eject tape when full' setting is turned off, so why is the tape being ejected?

Is this simply a case of increasing the compression setting in Acronis?

Many Thanks in Advance.

The following logs entries are generated:

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Log Entry Details
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Type: Information
Date and time: 2/27/2013 9:00:07 PM
Backup plan: [None]
Task: [None]
Code: 20,250,627(0x1350003)
Module: 309
Owner: Acronis Agent User
Message:
Command 'Backing up' is running.
--------------------

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Log Entry Details
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Type: Information
Date and time: 2/27/2013 9:00:16 PM
Backup plan: [None]
Task: [None]
Code: 66,039(0x101F7)
Module: 1
Owner: Acronis Agent User
Message:
Analyzing partition '1-0'...
--------------------

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Log Entry Details
--------------------
Type: Information
Date and time: 2/27/2013 9:00:16 PM
Backup plan: [None]
Task: [None]
Code: 66,544(0x103F0)
Module: 1
Owner: Acronis Agent User
Message:
Create Full Backup From: Disk '1' To: Compression: Normal Exclude: Files matching mask Match criterion: *\System Volume Information\*{3808876B-C176-4e48-B7AE-04046E6CC752}; ... Ask before overwriting tape: No
--------------------

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Log Entry Details
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Type: Information
Date and time: 2/27/2013 9:00:19 PM
Backup plan: [None]
Task: [None]
Code: 22,020,227(0x1500083)
Module: 336
Owner: Acronis Agent User
Message:
Tape 'Tape w/o barcode' has been mounted to drive 'QUANTUM ULTRIUM 3 2181 (PW1011AME50059)'.
Additional info:
--------------------
Error code: 131
Module: 336
LineInfo: ff11bd907bd9e090
Fields: $module : C:\Program Files\Acronis\BackupAndRecovery\DiskBundle.dll
Message: Tape 'Tape w/o barcode' has been mounted to drive 'QUANTUM ULTRIUM 3 2181 (PW1011AME50059)'.
--------------------
--------------------

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Log Entry Details
--------------------
Type: Information
Date and time: 2/27/2013 9:00:20 PM
Backup plan: [None]
Task: [None]
Code: 6,553,600(0x640000)
Module: 100
Owner: Acronis Agent User
Message:
Media with label 'Tape w/o barcode' and barcode '' is being processed.
--------------------

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Log Entry Details
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Type: Information
Date and time: 2/27/2013 9:00:20 PM
Backup plan: [None]
Task: [None]
Code: 66,040(0x101F8)
Module: 1
Owner: Acronis Agent User
Message:
Pending operation 151 started: 'Creating partition image'.
--------------------

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Log Entry Details
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Type: Information
Date and time: 2/27/2013 9:00:30 PM
Backup plan: [None]
Task: [None]
Code: 66,042(0x101FA)
Module: 1
Owner: Acronis Agent User
Message:
Locking partition 0-0...
--------------------

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Log Entry Details
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Type: Information
Date and time: 2/27/2013 9:25:05 PM
Backup plan: [None]
Task: [None]
Code: 66,040(0x101F8)
Module: 1
Owner: Acronis Agent User
Message:
Pending operation 151 started: 'Creating partition image'.
--------------------

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Log Entry Details
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Type: Information
Date and time: 2/27/2013 11:32:25 PM
Backup plan: [None]
Task: [None]
Code: 22,020,226(0x1500082)
Module: 336
Owner: Acronis Agent User
Message:
Tape 'Tape w/o barcode' has been ejected.
Additional info:
--------------------
Error code: 130
Module: 336
LineInfo: ff11bd907bd9e0c1
Fields: $module : C:\Program Files\Acronis\BackupAndRecovery\DiskBundle.dll
Message: Tape 'Tape w/o barcode' has been ejected.
--------------------
--------------------

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Log Entry Details
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Type: Warning
Date and time: 2/27/2013 11:32:25 PM
Backup plan: [None]
Task: [None]
Code: 4,390,941(0x43001D)
Module: 67
Owner: Acronis Agent User
Message:
Insert a free media to library 'QUANTUM ULTRIUM 3 2181 (PW1011AME50059)' and click Retry; or click Cancel to break the operation.
Additional info:
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Error code: 29
Module: 67
LineInfo: 87a7a5e658247679
Fields: $module : C:\Program Files\Acronis\BackupAndRecovery\DiskBundle.dll
Message: Insert a free media to library 'QUANTUM ULTRIUM 3 2181 (PW1011AME50059)' and click Retry; or click Cancel to break the operation.
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