Duplicate Custom field: The field 'custom_field[fieldname]' was not recognized


Well-known member
So I added a few profile fields in the AdminCP, and then went to my personal details to fill these in. However, after filling them in, there's a little balloon next to the field showing an error "The field 'custom_field[fieldname]' was not recognized".

I tried using another field without any weird characters in it, but that had the same result.

See attachments.