All your reason code customization already in development system ? Its really strange. I did import to categorization schema to different clients and never had this kind of issues.
Are you using program CRM_ERMS_CAT_AS_IMPORT or IMPORT option through WEB UI option in IC MANAGER Role ?
Can you try using Application area options ( such as Application ID, Parameter) in the program during import? Did you find anything suspicious in RFC user trace ?