Hi Romuald,
now it's clear your issue.
Sorry I never used the admin task to update dims. Have you tried omitting this parameter?
Have you opened this package from SSIS? If so open it from bpc organize package list=>modify package =>view package=> advanced and see the values.
Try substituting these with
TASK(Admin_Task,APP,%APP%)
TASK(Admin_Task,USERID,%USER%)
If doesn't work please share the full text.
Kind regards
Roberto