What causes the 'DB Error: unknown error' after adding custom fields to a report in CiviCRM?
After turning on the debugging tool,it appears the field label was changed at some point and now cannot be referenced through reporting because the database is expecting the old field label and set name of the database cell to be referenced. It doesn't affect exporting the data though. If you have no access to the actual server then a solution is to create a new field and export and import all the data from the errant field to the new field, as far as I understand.