Try to check is the configuration of User/Pass for OCS connection to database is correct, beside i suggest first to do an import on the same version of OCS and then to do Upgrade to 2.6, or other way first do Upgrade from 2.4 to 2.6 and then export and import on new machine with 2.6 :) This is quite important to operate export/import on the same version OCS and database.