error:One or more authors don't exist

Hello,

I am trying to submit my first project to Project Hub

Everything is fine except that I am getting the error "error:One or more authors don't exist" when I am trying to add a second member to the project.

In the Section "The Team", I click "+ add a member", then I "type the member name", and then I click "add member".
If I type a member name that does not exist then I rightfully get the message "this member does not exist" when à click "add member".
When I type the member name of my coauthor that does exist, clicking "add member" generates no error message and the member name is added to the list of members. So far so good.
However, the error message appears when I click "save changes" or "go to preview"
The member name contains only letters (lower case and upper case).

This prevents me from accessing the preview page and submitting the project.

Can you please help ?