Shopify admin internal issue

Some of our customers, experience a strange issue on the Customify product customizer page.

They have seen an error message on the customizer page entitled 

Liquid error: internal 

This error appears when the shop owner or the staff back-end logged in, and only logged in back-end user can see this error message.

The end customer will not see this error message.

We have reported this issue to the Shopify technical support.