TI 2014 Premium: Error message during backup but validation says OK

I have recently developed a problem with receiving an error message during backup creation (incremental) saved to my Drobo NAS that states the operation completed with errors. However, when I ran Validate, it is a valid. Can anyone tell from the posted information below what the problem might be? I should state that is problem only recently developed after having been running for a number of weeks without a problem.

Thanks

1 Information 12/19/2014 10:03:02 PM Backup reserve copy attributes: format tib; need_reserve_backup_copy false;
2 Information 12/19/2014 10:03:02 PM Operation Thunderbird started by schedule.
3 Information 12/19/2014 10:03:02 PM Operation description: Stage Description.
4 Information 12/19/2014 10:03:11 PM Backup reserve copy attributes: format tib; need_reserve_backup_copy false;
5 Information 12/19/2014 10:03:11 PM Priority changed to Low.
6 Information 12/19/2014 10:03:15 PM Create Backup Archive From: E:\Thunderbird\
To file: "Thunderbird_inc_b5_s6_v1.tib" Compression: Normal Exclude: System files
7 Information 12/19/2014 10:03:17 PM Writing incremental version to file: Thunderbird_inc_b5_s6_v1.tib
8 Information 12/19/2014 10:07:05 PM The following backups have been successfully created: "\\Drobo5N\Public\BACKUPS\Email\Thunderbird_inc_b5_s6_v1.tib"
9 Information 12/19/2014 10:07:05 PM Validate Backup Archive Location: "\\Drobo5N\Public\BACKUPS\Email\Thunderbird_inc_b5_s6_v1.tib"
10 Information 12/19/2014 10:28:20 PM Consolidate Backup Archive Location: "\\Drobo5N\Public\BACKUPS\Email\Thunderbird_inc_b5_s6_v1.tib" Destination: "\\Drobo5N\Public\BACKUPS\Email\Thunderbird_inc_b5_s6_v1_623E8B94-E406-491C-8B41-E7479EB894AB.tib"
11 Information 12/19/2014 10:28:20 PM Consolidate Backup Archive Location: "\\Drobo5N\Public\BACKUPS\Email\Thunderbird_inc_b5_s6_v1.tib" Destination: "\\Drobo5N\Public\BACKUPS\Email\Thunderbird_inc_b5_s6_v1_623E8B94-E406-491C-8B41-E7479EB894AB.tib"
12 True Image 12/19/2014 10:28:20 PM Failed to exclusively access backup Z:\BACKUPS\Email\Thunderbird2014-11-24WIN7Thunderbird13_inc_b2_s5_v1.tib during consolidation. Make sure the backup file is not opened in other applications.
13 True Image 12/19/2014 10:28:20 PM Error occurred while opening the file.
14 True Image 12/19/2014 10:28:20 PM The system cannot find the path specified
15 True Image 12/19/2014 10:28:20 PM Failed to open data stream. Try to remove the backup from the list, add it again by clicking 'Browse for backup' and recreate the backup settings.
16 True Image 12/19/2014 10:28:20 PM Failed to exclusively access backup Z:\BACKUPS\Email\Thunderbird2014-11-24WIN7Thunderbird13_inc_b2_s5_v1.tib during consolidation. Make sure the backup file is not opened in other applications.
17 True Image 12/19/2014 10:28:20 PM Failed to exclusively access backup Z:\BACKUPS\Email\Thunderbird2014-11-24WIN7Thunderbird13_inc_b2_s5_v1.tib during consolidation. Make sure the backup file is not opened in other applications.
18 True Image 12/19/2014 10:28:20 PM Error occurred while opening the file.
19 True Image 12/19/2014 10:28:20 PM The system cannot find the path specified
20 True Image 12/19/2014 10:28:20 PM Failed to exclusively access backup Z:\BACKUPS\Email\Thunderbird2014-11-24WIN7Thunderbird13_inc_b2_s5_v1.tib during consolidation. Make sure the backup file is not opened in other applications.
21 True Image 12/19/2014 10:28:20 PM Failed to exclusively access backup Z:\BACKUPS\Email\Thunderbird2014-11-24WIN7Thunderbird13_inc_b2_s5_v1.tib during consolidation. Make sure the backup file is not opened in other applications.
22 True Image 12/19/2014 10:28:20 PM Failed to exclusively access backup Z:\BACKUPS\Email\Thunderbird2014-11-24WIN7Thunderbird13_inc_b2_s5_v1.tib during consolidation. Make sure the backup file is not opened in other applications.
23 True Image 12/19/2014 10:28:20 PM Failed to exclusively access backup Z:\BACKUPS\Email\Thunderbird2014-11-24WIN7Thunderbird13_inc_b2_s5_v1.tib during consolidation. Make sure the backup file is not opened in other applications.
24 True Image 12/19/2014 10:28:20 PM Error occurred while opening the file.
25 True Image 12/19/2014 10:28:20 PM The system cannot find the path specified
26 True Image 12/19/2014 10:28:20 PM Operation has completed with errors.