Also got the same issue.
Happens for me at 3.3.15
Found the problem and Ninja forms introduced a new hook ninja_forms_admin_import_template_capabilities
You may solve the permission issue yourself until an update is done
add_filter( 'ninja_forms_admin_import_template_capabilities', 'penf_manage' );
I’ve put the code in functions.php of my theme. Another solution is to create a wrapper plugin with this filter. But for now functions.php seems the easiest way to add the missing functionality.
