Copy fails with "ORA-01034: ORACLE not available", "EXP-00056" and / or "ORA-27101: shared memory (4331041)

When running copy to sync the target table, the copy status command shows the following errors:

sp_ctrl >copy status

Job ID : 245
Host : host_name
Started : 07-JUL-09 17:38:19
Job Type : Copy
Status : Failed
ORA-01034: ORACLE not available
See sync_svr_245 log file for details

ID Tablename Total Rows %Comp Status Status Time Total Time
------ ------------------------------------ ---------- ----- ---------- ----------- ----------
1 OWNER.TABLENAME 46328065 Failed N/A 0:03

The following errors are observed in the sync server or sync client log (source and target copy log respectively):

Wed Jun 03/13:43:57.025:: ERROR: ORA-1034: ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist

Or

Wed Jul 01/13:39:01.797:: 003: Error/Warning Detected: EXP-00056: ORACLE error 1034 encountered
Wed Jul 01/13:39:01.798:: 003: Error/Warning Detected: ORA-01034: ORACLE not available
Wed Jul 01/13:39:01.798:: 003: Error/Warning Detected: ORA-27101: shared memory realm does not exist
Wed Jul 01/13:39:01.798:: 003: Error/Warning Detected: EXP-00005: all allowable logon attempts failed
Wed Jul 01/13:39:01.798:: 003: ERROR: Stopping export thread - ORA-01034: ORACLE not available at sync/svr/sync_export_thread.cpp:630