Activity that doesn't plan with certain users

Hello
We have problems with "Stains" Activities and only with certain users. (especially with new users), they don't appear in the 'Agenda' module, but they do appear well in the 'Planning'.

we created "Stains" Activity with an X Manager and assigned User X with a time of 7H on that spots.
We rotate the system so that the activity appears in the Calendar, but it doesn't work. (By zooming in on activity, nothing appears in the "Planned Date" area)

We do the same actions, with the same dates and with a User Y that exists for a long time is there, it works.
Since this Activity appeared, we tried to put User X in charge and assigned it, we ran the system and the activity disappeared from the Schedule and the "Planned Date" disappeared on the activity!

it was deduced that this may have come from the creation of the user!
So we created a Z User using the copy icon that is on user Y that works. We recreate an Activity in pheasants the same actions, but it does not work!

We do not see where is the problem, do you have a solution?

Kind regards