XAER_RMFAIL : Resource manager is unavailable

Could not invoke operation 'CheckMachineExist' due to:

BINDING.JCA-11812
Interaction processing error.
Error while processing the execution of the CONFIGDB.CHECKMACHINEEXISTS API interaction.
An error occurred while processing the interaction for invoking the CONFIGDB.CHECKMACHINEEXISTS API. Cause: Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.1.3.v20110304-r9073): org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: java.sql.SQLException: Unexpected exception while enlisting XAConnection java.sql.SQLException: XA error: XAResource.XAER_RMFAIL start() failed on resource 'PIPPO_osbpr1do': XAER_RMFAIL : Resource manager is unavailable
oracle.jdbc.xa.OracleXAException
        at oracle.jdbc.xa.OracleXAResource.checkError(OracleXAResource.java:1616)
        at oracle.jdbc.xa.client.OracleXAResource.start(OracleXAResource.java:336)
        at weblogic.jdbc.wrapper.VendorXAResource.start(VendorXAResource.java:50)
        at weblogic.jdbc.jta.DataSource.start(DataSource.java:729)
        at weblogic.transaction.internal.XAServerResourceInfo.start(XAServerResourceInfo.java:1231)
        at weblogic.transaction.internal.XAServerResourceInfo.xaStart(XAServerResourceInfo.java:1164)
        at weblogic.transaction.internal.XAServerResourceInfo.enlist(XAServerResourceInfo.java:285)
        at weblogic.transaction.internal.ServerTransactionImpl.enlistResource(ServerTransactionImpl.java:522)
        at weblogic.transaction.internal.ServerTransactionImpl.enlistResource(ServerTransactionImpl.java:449)
        at weblogic.jdbc.jta.DataSource.enlist(DataSource.java:1599)
        at weblogic.jdbc.jta.DataSource.refreshXAConnAndEnlist(DataSource.java:1503)
        at weblogic.jdbc.jta.DataSource.getConnection(DataSource.java:446)
        at weblogic.jdbc.jta.DataSource.connect(DataSource.java:403)
        at weblogic.jdbc.common.internal.RmiDataSource.getConnection(RmiDataSource.java:364)
       at org.eclipse.persistence.sessions.JNDIConnector.connect(JNDIConnector.java:126)

This might well be because your Oracle DB has hit the maximum number of processes.

select count(*) from v$process;

show parameter session

NAME                                               TYPE        VALUE                                                                                                
-------------------------------------------------- ----------- ---------------------------------------------------------------------------------------------------- 
java_max_sessionspace_size                         integer     0                                                                                                    
java_soft_sessionspace_limit                       integer     0                                                                                                    
license_max_sessions                               integer     0                                                                                                    
license_sessions_warning                           integer     0                                                                                                    
session_cached_cursors                             integer     50                                                                                                   
session_max_open_files                             integer     10                                                                                                   
sessions                                           integer     480                                                                                                  
shared_server_sessions                             integer                                                                                                          

show parameter processes

-------------------------------------------------- ----------- ---------------------------------------------------------------------------------------------------- 
NAME                                               TYPE        VALUE                                                                                                
-------------------------------------------------- ----------- ---------------------------------------------------------------------------------------------------- 
aq_tm_processes                                    integer     1                                                                                                    
db_writer_processes                                integer     1                                                                                                    
gcs_server_processes                               integer     2                                                                                                    
global_txn_processes                               integer     1                                                                                                    
job_queue_processes                                integer     1000                                                                                                 
log_archive_max_processes                          integer     4                                                                                                    
processes                                          integer     300  

You can change it with
alter system set processes=300 scope=spfile;