Symbos error 18

[Vitop69]

CPC 464 with DDI5 + Fd1 floppy drive B.

symbos starts well from ddi5, standard app ok.
when I try to launch any other application like pac-man, symbsnap I always get this error:

Error while loading:
A disc error occurred during the loading process (error code 18)

From shell i obtain error in file organization.

what am I doing wrong?
thanks for your help.