Trouble with User Registration/Woo Commerce Checkout on Masked Domain

I’m working on a client’s site right now with a multisite installation I didn’t install. However, it seems to be working correctly and you are able to create new sites on it, but I’ve run into a snag I’m hoping someone here can shed some insight into.

The client already had several network sites created, each with a great deal of content; I’m charged with building out one of them. This site is running Sensei with WooCommerce. The client has also forwarded SubdomainName.com from GoDaddy with masking to clientsite.com/subdomain and that, too, seems to be working as it should, except-

Everything works perfectly when we go to https://clientsite.com/subdomain and try to run a test purchase. The cart, the checkout process, the confirmation, everything works flawlessly.

But when looking at the site through SubdomainName.com, we cannot register or checkout. It goes to a blank page.

In otherwords, clientsite.com/subdomain/checkout is the shopping cart to which WooCommerce redirects when we click “Go to Checkout”. But when we do the same process and try to go to SubdomainName.com/checkout we get a blank page.

I’ve tried this with the twentyfifteen theme and no other plugins except WooCommerce enabled, and the same issue happens.

Am I correct in surmising that this may have something to do with the way user admin and registration were configured for this particular network? Isn’t there a setting where you can disable registration through subdomains or something like that? It’s been a while since I set this up, admittedly, and I can’t see where to change that in the network dashboard, so does anyone have any idea how to fix this?

Sorry so long and many thanks if anyone has any insight on this.