We were experiencing this same issue for some users, and Support helped resolve this by:

Gear icon > Manage Users > Edit (for affected users) >
change "Do you want this user to edit company info?"

from "no" to "yes"

This then allowed them to use the Custom templates again. All users had been using these templates for 3+ years, but they just stopped working this week for some reason. But making this change for those users now allows them to use the Custom form templates again