Out of memory (Failed on request size 24)

PostgreSQL 8.0.3 running on AIX 5.3 (same thing happens on 5.1 though).
DBMS was running fine for some months but now one of the databases isn't accessible. Any help would be greatly appreciated.

DBMS starts up fine, but any operation on the files database (psql files, vaccumdb files, pgdump files) yields the same result. The client responds with

> psql files
DEBUG: InitPostgres
DEBUG: StartTransaction
DEBUG: name: unnamed; blockState: DEFAULT; state: INPROGR, xid/subid/cid: 626915/1/0, nestlvl: 1, children: <>
psql: FATAL: out of memory
DETAIL: Failed on request of size 24.

The server log file contains the following

LOG: database system was shut down at 2006-11-14 04:38:54 EST
LOG: checkpoint record is at 0/19A82450
LOG: redo record is at 0/19A82450; undo record is at 0/0; shutdown TRUE
LOG: next transaction ID: 626912; next OID: 54355
LOG: database system is ready
DEBUG: proc_exit(0)
DEBUG: shmem_exit(0)
DEBUG: exit(0)
DEBUG: reaping dead processes
DEBUG: forked new backend, pid=168724 socket=7
DEBUG: postmaster child[168724]: starting with (
DEBUG: postgres
DEBUG: -v196608
DEBUG: -p
DEBUG: files
DEBUG: )
DEBUG: InitPostgres
DEBUG: StartTransaction
DEBUG: name: unnamed; blockState: DEFAULT; state: INPROGR, xid/subid/cid: 626912/1/0, nestlvl: 1, children: <>
TopMemoryContext: 42416 total in 4 blocks; 12312 free (2 chunks); 30104 used
TopTransactionContext: 2145378304 total in 266 blocks; 928 free (14 chunks); 2145377376 used
PortalMemory: 0 total in 0 blocks; 0 free (0 chunks); 0 used
CacheMemoryContext: 516096 total in 6 blocks; 178752 free (10 chunks); 337344 used
pg_operator_oid_index: 1024 total in 1 blocks; 840 free (0 chunks); 184 used
pg_amproc_opc_proc_index: 1024 total in 1 blocks; 736 free (0 chunks); 288 used
pg_amop_opc_strat_index: 1024 total in 1 blocks; 736 free (0 chunks); 288 used
pg_index_indexrelid_index: 1024 total in 1 blocks; 840 free (0 chunks); 184 used
pg_attribute_relid_attnum_index: 1024 total in 1 blocks; 744 free (0 chunks); 280 used
pg_class_oid_index: 1024 total in 1 blocks; 840 free (0 chunks); 184 used
pg_amproc_opc_proc_index: 1024 total in 1 blocks; 736 free (0 chunks); 288 used
pg_amop_opc_strat_index: 1024 total in 1 blocks; 736 free (0 chunks); 288 used
pg_class_relname_nsp_index: 1024 total in 1 blocks; 744 free (0 chunks); 280 used
MdSmgr: 8192 total in 1 blocks; 7808 free (0 chunks); 384 used
DynaHash: 8192 total in 1 blocks; 5936 free (0 chunks); 2256 used
Operator class cache: 8192 total in 1 blocks; 1968 free (0 chunks); 6224 used
smgr relation table: 24576 total in 2 blocks; 16256 free (5 chunks); 8320 used
Portal hash: 8192 total in 1 blocks; 4032 free (0 chunks); 4160 used
Relcache by OID: 8192 total in 1 blocks; 928 free (0 chunks); 7264 used
Relcache by name: 24576 total in 2 blocks; 14208 free (5 chunks); 10368 used
LockTable (locallock hash): 24576 total in 2 blocks; 16272 free (6 chunks); 8304 used
ErrorContext: 8192 total in 1 blocks; 8160 free (7 chunks); 32 used
FATAL: out of memory
DETAIL: Failed on request of size 24.
DEBUG: proc_exit(0)
DEBUG: shmem_exit(0)
DEBUG: exit(0)
DEBUG: reaping dead processes
DEBUG: server process (PID 168724) exited with exit code 0