Ok. My apologies.
Received this from Macrium:
This issue is caused by Avast, we have reached out to them for assistance but have not had a reply.
Version 7 of Reflect will solve the issue. The issue has been identified and a workaround has been documented. Please see this link:
Have to agree this is 100% a Avast problem ,all other av’s work with Macrium.
This is not true, well at least for me. 
Turning off Avast’s self-defense module (followed by a reboot) allowed successful completion of backups using the Task Scheduler. Backups work fine with the self-defense module active if they are run manually. The backup problem only occurs using the Task Scheduler.
My issue did not start until the next scheduled backup after I upgraded to version 17.1.2286 (build 17.1.3394.42). I don’t know if this self-defense module is a new feature, was turned on in the new release, or was upgraded.
Deactivate the self-defense module as described by member gtdurham or go to:
http://forum.macrium.com/FindPost10821.aspx
I opened a support ticket with Macrium and they gave me the solution described in the post as well.
They also told me that “We have reached out to Avast to work with them on a resolution but they, unfortunately, declined”.
I’m completely disappointed that Avast chose not to work with them assuming that that statement is true (which I have no reason to believe otherwise).
Selfdefense module isn’t new and is meant to prevent third party changes from making changes to anything Avast.
Which part of Macrium is responsible for attempting to make a change in something to do with Avast ???
I’m also reporting this to Avast. Hopefully a developer will chime in.
i’m not sure what Macrium could possibly be doing as far as making a change in something to do with Avast. it’s simply a backup program and it is fairly lightweight.
besides if it was trying to make a change to Avast why does it work when i run the program manually? the error only occurs when it is invoked via the Task Scheduler.
A question for the developers to answer. I’m an Avast user just like you.
I actually don’t use Macrium. I use the built in Windows image creation tool.
Even with the update provided by Macrium Reflect, the scheduled backups still don’t run.
Macrium have released an update:
http://forum.macrium.com/Topic10765-1.aspx
Is the issue still happening?
Still fails for me, but the error code has changed to 0x13b
Can you explain in detail how to reproduce it?
You should consider the latest avast release (not beta) https://forum.avast.com/index.php?topic=197955.0. That also what is meant to be a fix.
No, we know that Avast has a problem to reproduce this.
That is why Alikhan asked above.
Greetz, Red.
Then hopefully with two ends of the issue releasing fixes it should be OK, unless of course if the changes introduce other issues
I have the latest avast [ 17.2.2288 / 27-Feb-17 ]
And I’m running Macrium 6.3.1745
To reproduce the error,
–I start Macrium,
–goto the ‘scheduled backup tab’
–Right click on ‘Differential’ on my backup job
–Select ‘Run now’
A pop-up box briefly flashes on the screen (Way,way to fast to read) and the ‘Last run time’ column updates and ‘0x13b’ appears the the ‘Last result’ column.
Nothing appears under the ‘Log’ tab.
If I disable the Avast ‘Self defense module’ and reboot, the same process starts the backup job running.
Can confirm it. Previously it gave 0x1f6 Error, now it returns with 0x13b Error 
Running the latest possible Macrium Reflect Free Edition 64bit 6.3 build 1745 and avast 17.2.2288 (17.2.3419.0)
Is the issue solved with Reflect scheduled backup jobs?
I would like to reinstall Avast Internet Security, but this issue is a deal-breaker for me
Suggest you wait till the next release is announced.
I am also getting the 0x13b error message .You said wait for a new release… Is that a new release of AVAST or Marcium?
I am at Marcium 6.3.1745 and AVAST 17.3.2291
https://forum.avast.com/index.php?topic=200322.0
There’s a fix for this in the latest BETA.
It would be helpful if you could test and let us know if it works.