FINALLY solved the problem. In a nutshell, M5v3 cannot find samples from M5v2 programs. The M5v2 programs have to be *re-loaded* manually into M5v3 from .m5p files previously saved on hard disk, then *re-saved* from M5v3 to replace the M5v2 programs with new M5v3 programs. Then all is fine again.
