Error: Failed to create base group for project

[chrisber2]

Trying to upgrade from PF3 to PF4 with the migration tool.
Tried twice already and got an error each time at the same stage :
at the level : "8. Migrating Project Access"
I get the message at 91% done : "Error: Failed to create base group for project"
What do do ?
How can I know what project blocks.
If I find it, should I just suppress it ?
Thanks for your help.