Verify the values from sqlnet.ora and tnsnames.ora are set correctly.
The sqlnet.ora had the parameter 'NAMES.DEFAULT_DOMAIN = ' configured.
When this is the case then the 'tnsnames' entries for the instance must be qualified with the domain name as identified in the sqlnet.ora file.
Entry in sqlnet.ora:
NAMES.DEFAULT_DOMAIN = xx.yyyyy.com
entry in tnsnames.ora:
STATP.xx.yyyyy.com = (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP)(HOST = kdxxxp03)(PORT = 1521)) )
(CONNECT_DATA = (SERVICE_NAME = statp) ) )
If the sqlnet.ora file does not use the domain parameter then the entries in the tnsnames file cannot use the 'domain' name as part of the alias name.