Your lead is missing a field mapping for the fields.

Hey,

Salesforce is giving me a bit of a headache today! For some reason I get the following error message when I try to convert a lead to an opportunity (they convert fine if I don't create an opportunity). 

Your lead is missing a field mapping for the fields. For help, talk with your admin or see Map Custom Lead Fields for Lead Conversion in Salesforce Help.

I've checked the mappings for both of the custom fields and the field types are identical, I've also checked picklist values for lead source etc. and ensured they're all available on the record types. I've tried converting in a sandbox and in production, in classic and in lightning, and nothing is giving me any more clues at to what the issue is. I'm not sure what else to try so I'm hoping someone can help on here.