It could be due to a wide array of problems. There is a Note in the metalink which talks about the possible scenarios. Note: 1003596.6

Check to make sure that you have your LD_LIBRARY_PATH, had been set correctly. Were you able to make a normal sqlplus connection to the database? How do you pass the SID to chich the oracle has to make connection to? Check your oratab file to make sure the paths had been set correctly.

Sam