Error opening a terminal session as another user

My database server is still running under HP-UX 11 with Oracle 8.1.7.4.0. When I open a terminal session with the 'oracle' user, I can start SQL*Plus and make a connection with the user system. Then I make a new terminal connecting with another user and start SQL*Plus with the connected user system. Then I get Oracle error ORA-03113 end of communication channel. No logs will be written to the log directories.
If you can perform such an operation when you sign on to the server as the 'oracle' user and you cannot perform this operation signed on to the server as another user, then this is usually because you do not have sufficient permissions as the non-oracle user to perform that operation. First, make sure that all of your environment variables have been set. You need to correctly set ORACLE_HOME, ORACLE_SID, and LD_LIBRARY_PATH, at a minimum. If the correct settings of the environment variables are not resolving your issue, then it is likely that the "sticky bit" on the Oracle executable file has not been set. To do this, sign on as oracle and issue the following:

chmod 6755 $ORACLE_HOME/bin/oracle