How to back up to a VeraCrypt container using Windows 10 built-in tools?

Background: I am running Windows 10 Pro 1709. My C: drive is encrypted with BitLocker. I have an ASUS RT-AC55U with an 8TB HDD attached to its USB 3.0 port. My goal is to use part of that HDD as an encrypted space for my backups—i.e., mine will be the only PC with the container mounted—while allowing the rest of the HDD to be accessible to other users on the network. To that end, I created a large VeraCrypt NTFS container on the network-attached HDD: Now I have mounted that container as the lettered drive B: and added some files from another source: I am trying to use File History and System Image Backup with the drive B:.

Problem: I can point File History at B:, but it will stop backing up my C: drive very shortly into the process. Subsequently, it will warn me that my data is not yet backed up while showing that the backup is a few GB in size:

Further attempts to run File History do not increase the backup size nor make the warning go away. Additionally, when I go to perform a system-image backup, the tool does not even list B: as a possible target, giving STATUS_WAIT_1 (0x80070001):

Things I've tried: Using a VeraCrypt container of a different format (doesn't work), backing up to an unencrypted external HDD (works but doesn't fit my needs), BitLocker-unencrypting my C: drive (makes no difference), plugging the HDD directly into my computer instead of going over the network (makes no difference), using a smaller container (makes no difference).

While I hope for a solution to this specific problem, I'd be happy to entertain answers that discuss alternate ways of meeting my goals.