One of my backups gives error when all should be OK

I have a disk backup that I run every two weeks on one of my computers. When it starts automatically, it always gives a write error when first trying to access the backup file on the NAS. If I come in and immediately thereafter and start it manually, all is OK. The NAS logs show a successful log in to the NAS so the credentials are OK.
All my other tasks on the machine have no problem accessing the NAS. I have identical tasks on my other machine and they all run fine.
Here's the relevant bit from the log.The file is the one that is to be created by this backup.
2018-01-25T17:00:08:336-08:00 2996 I000101F8: Pending operation 173 started: 'Creating partition image'.
2018-01-25T17:00:12:533-08:00 9676 I00640000: Writing full version to file: C Drive_full_b10_s1_v1.tib
2018-01-25T17:00:12:995-08:00 9676 E01E50023: Error 0x1e50023: Cannot access backup file \\NASBox\Acronis\Asus\Disk Images\C Drive\C Drive_full_b10_s1_v11.tib. Please ensure that you entered the correct credentials for the network share.
And what it looks like in the log when I start it manually:
2018-01-25T18:07:36:300-08:00 924 I000101F8: Pending operation 173 started: 'Creating partition image'.
2018-01-25T18:07:39:433-08:00 10052 I00640000: Writing full version to file: C Drive_full_b10_s1_v1.tib
2018-01-25T18:07:41:132-08:00 924 I000101F8: Pending operation 173 started: 'Creating partition image'.
2018-01-25T18:18:52:735-08:00 924 I000101F8: Pending operation 173 started: 'Creating partition image'.
2018-01-25T18:19:03:294-08:00 924 I000101F8: Pending operation 173 started: 'Creating partition image'.
2018-01-25T18:19:03:342-08:00 924 I00640000: The following backups have been successfully created:<endl/>"\\NASBox\Acronis\Asus\Disk Images\C Drive\C Drive_full_b10_s1_v1.tib"
So what is weird is that is wants to read the file ..._b10_s1_v11.tib. Why v11? This a a full backup every time, no incremental or differential. It keeps not more that 2 in the history.
Is there something I could do to clean it up?
It is an easy enough backup to just recreate, although figuring out the problem is an exercise in understanding.
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Steve, this is set up as a Full backup with validation and with cleanup set to keep no more than 2 full versions. On the NAS, all looks perfect. The folder contains the two expected files ... C Drive_full_b10_s1_v1.tib and C Drive_full_b11_s1_v1.tib, and nothing else.
Validation is done on every backup and passes well.
What seems odd is that the failure only occurs when the task is started by schedule, but not when the task is run manually.
I checked the script file and see nothing amiss there. I also ran the Scheduler Manager and all appears OK there.
In checking past history, I did find that on 12/28/2017 two things happened. First is that I updated the machine from Windows 7 Pro to Windows 10 Pro. Second is that when the task ran that evening I manually stopped it. I can't remember why I did that, but it has been since that time that the failures have happened. No clue in there as to what that v11 thing is all about.
I think what I will do is clone the task and see if that fixes it.
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If it were me I would ZAP the task and recreate it anew. That would get rid if the bad task from the database and in turn fix the issue.
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Update... I cloned the task. But the error still exists in the new task. I have now created a completely new task from scratch. Since by schedule it only runs twice a month, I will need to wait until March 8th to see if it succeeds by schedule.
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I have some interesting new information.
This same problem has just appeared on my second machine... here is the fault in the log file.
2/27/2018 4:55:05 PM: -08:00 8656 I00640000: Writing full version to file: C Drive_full_b12_s1_v1.tib
2/27/2018 4:55:06 PM: -08:00 8656 E01E50023: Error 0x1e50023: Cannot access backup file \\NASBox\Acronis\Asus2\Disk Images\C Drive\C Drive_full_b12_s1_v11.tib. Please ensure that you entered the correct credentials for the network share.
This backup task does a full backup every time and keeps no more than two versions... same on both machines. It has worked just fine up until now. On my other machine, the problem started occurring when trying to create C Drive_full_v9_s1_v1. Oddly, the failure only happens when running the backup on schedule. Manually starting it works just fine.
As a programmer, I am now nearly convinced that there is a string handling problem in the program which has appended an extra "1" on the end of the file name. I will report it as a bug to Acronis as soon as I can figure out how to write it up.
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Bruno, good spot if this is proved to be correct and the extra '1' is the bug here!
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Acronis Links : Acronis Scheduler Manager : Acronis VSS Doctor : Backup Archive Compatibility : Cleanup Tool (All versions) : Cloning Disks : Contact Acronis Support : Difference between Backup and Disk Clone : Repair program / settings
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Hello BrunoC,
May I ask you to send us Acronis system report from the affected PC via Feedback along with a couple of screenshots showing your settings ans how the backup chain looks in the target directory?
Thank you in advance!
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Best regards,
Ekaterina Surkova | Forum Moderator
Information provided AS-IS with no warranty of any kind.
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In reply to Hello BrunoC,… by Ekaterina

Hello Ekaterina,
I have already opened a support case. I just generated a system report and screen shots as you've requested and sent it via feedback. I hope it helps.
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Further information...
As mentioned above on 2/23/2018, I created a completely new backup task on my wife's PC. It was configured just like the failing task but I changed the name of the backup so as to avoid file name issues. I created the first backup manually.
When it was time to run the next backup by schedule, it failed again. So I changed the task to use a different backup destination folder on the NAS and reset the schedule to start in a few minutes. It then succeeded to run on schedule. It is next scheduled to run in two weeks so we'll see what happens then.
I have provided the information to the team for the support case.
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Read about the MVP Assistant via this forum topic.
Download the latest version of the MVP Assistant 2.6.2
Download the previous version of the MVP Assistant 2.5.2
Acronis Links : Acronis Scheduler Manager : Acronis VSS Doctor : Backup Archive Compatibility : Cleanup Tool (All versions) : Cloning Disks : Contact Acronis Support : Difference between Backup and Disk Clone : Repair program / settings
MVP Assistant (Log Viewer) latest version see pinned topic in ACPHO forum page.
Acronis True Image User Guides available from Product Documentation page.
imTranslator for quick translation of language posts.
Please do NOT send private messages about urgent issues - open a forum topic instead where you will get a quicker response!!