indeed as Gerald mentionned. I had the same kind of problems some years ago.
Below some more info.
/*
Other locking schemes from dbinfo.ch // LOCK SCHEMES: RDDI_LOCKSCHEME, DBI_LOCKSCHEME
#define DB_DBFLOCK_DEFAULT 0
#define DB_DBFLOCK_CLIPPER 1 // default Cl*pper locking scheme
#define DB_DBFLOCK_COMIX 2 // COMIX and CL53 DBFCDX hyper locking scheme
#define DB_DBFLOCK_VFP 3 // [V]FP, CL52 DBFCDX, SIx3 SIXCDX, CDXLOCK.OBJ
#define DB_DBFLOCK_HB32 4 // Harbour hyper locking scheme for 32-bit file API, table size max 4GB
#define DB_DBFLOCK_HB64 5 // Harbour hyper locking scheme for 64-bit file API, table size no limit
#define DB_DBFLOCK_CLIPPER2 6 // extended Cl*pper locking scheme NTXLOCK2.OBJ
*/
SET(_SET_DBFLOCKSCHEME, DB_DBFLOCK_HB32 ) // 4 = max 4GB