Yes, I have tried with several administrators.
Admin ID1 is associated with ID2 Phpbb.
No, I don’t have to uninstall it, the problem occurs if I try to disable or uninstall any other plugin. Only without “WP w3all phpBB 2.3.9”, the problem does not occur.
However it is likely that there is a conflict with other plugins or with the theme. I will try to investigate better with more time.
For now, I solved it by downgrading to version 2.1.5 and the problem is no longer present for the moment.
Thanks.