pid 8300 ( 1%ä 1%ä 1%ä 1%ä 1%ä): assert (`curproc->stack_magic == STACK_MAGIC')
failed at line 503 of ../sig_mach.c
FATAL ERROR. You must reboot the system.
Did you try also 030 version? As I suggested earlier, these are exact kind of errors which you would spot with broken memory in CT60. FreeMiNT is much more sensitive to defects in RAM than TOS for example.