1.) 	first check the NLS settings

 	select * from nls_database_parameters;
	
	is the result UTF8?
	then
	You are NOT using SQL*Net version 2.3.4 and higher.  SQL*Net version 2.3.4
	and higher is required to be able to connect to a Unicode UTF8 database.

2.)	In extreme circumstances, this could indicate that a shared memory segment 
	used by Oracle is corrupted.  You will have to shut down the database, 
	possibly using the abort option.  Make sure all semaphores have been 
	released by using the IPCS command on Unix.  Oracle uses semaphores to 
	control concurrency between all of the background processes (pmon, smon, 
	drwr, lgwr, and oracle shadow processes).  Semaphores are also used to 
	control Two-Task communication between the user process and the shadow 
	process.