@Malachi,
I can’t give you a solution, but maybe you can think of the difference between your system and the others saying it works for them?
If you think it is Avast causing the problem (which is still a possibility, why not), then maybe, just for testing and not as a recommendation, you could try excluding the relevant folders from Avast?
Exclude the destination folder of your backups (and, if you can, the source folder too) in every shield of Avast AND in the settings → exclusions list. (I’m not recommending this as a permanent exclusion, but only for help you test the issue.)
Also, how much free space you have, both in your source partition and in you destination volume? This is usually a problem affecting backups.
How much disk space you are backing up, and how much do you have reserved for VSS?
Any other resident program (other than Avast)? Also, older security tools that you used to use that might have leaved remnants might need you to run their respective clean removal utilities from Windows Safe Mode.
You could check that, at the time you have scheduled the backup to run, there are no other processes intensively using resources (like a schedule Avast’s scan, or Windows Update, or scandisks/checkdisks, or…).
Those questions might help you find the difference between your system and others, and why in your case it doesn’t work.
In addition, you could try updating Avast to the latest beta, which has some bug fixes too.