Error on activity import: "'1' is not a valid option for field custom_42". Can I find which field this is referring to through the admin interface?
You may find that you can go to this url and find the field you are looking for
/civicrm/admin/custom/group/field/update?action=update&amp;reset=1&amp;id=

Or, to reverse engineer this, go to any existing custom field and mouse over the view/edit links and you should see the field ID along with the field group ID - i just dropped out the field group id to make the url look a little less intimidating.