You can export the a Custom Field by it's id instead of the name.
Step-by-step guide
Custom Field id : 123456
${customfield_[123456]}
Tip: To find the custom field id go to Issues - Custom Fields - Choose the custom field that you want and press edit:
Link for related documentation:
https://confluence.xpand-it.com/display/jiraxporter510/Mappings#Mappings-CustomFieldMappings