I don't know if it's here that i should post, but i've found a bug in ispconfig 3.1. Installtion is ok, but when i put in french it is impossible to add a client. Change into english, and it works well. That's all
Btw. It has already been reported and also fixed. https://git.ispconfig.org/ispconfig/ispconfig3/merge_requests/320
But this does not only apply to french. The rest is fixed here https://git.ispconfig.org/ispconfig/ispconfig3/merge_requests/321