"Internal Server Error" In Customize Settings Preview In Wordpress Sub-Sites

I'm currently having an issue where sub-sites on my Wordpress Multisite Install don't display the site preview when in Customize settings. See following image:

https://screencast.com/t/eq99Djox

I've noticed this is due to the Domain Mapping plugin being set to "Directly to mapped (primary) domain."

When I change it to "Mapped to original domain", the customizer preview works as it should!

I also created a test site and, if it's not mapped, it's customizer preview works as it should.

I also switched themes and as long as the site is mapped, the preview won't work.

Could someone please look at this for me. I'm not sure if it's to do with the Super Admin settings for the Domain Mapping plugin? Or something else?

I've support granted access to my site.