The database server cannot open the file that represents this table. You might have exceeded an operating-system limit on open files; if that is the case, you will have to reduce the number of tables that are included in the form. However, first make sure that a file exists for this table in the database directory (its name is database-name.dbs) and that your account has read access to it. The name of the table file is composed of table-name, a 3-digit number, and the suffix .dat.