this could be one of the reasons.. while ur doing sqlplus /, you obiviously have set ORACLE_HOME to right path.. and that ORACLE_HOME/network/admin has right entry for instance on server 2..

in your second case, you may be using some other oracle_home and corresponding tnsnames.ora has wrong entry..


another possibility is that your using private db links and in you have wrongly defined the entry.


rgds
abhay