Vendor registration error / request "?store-setup=yes" does not work…

Hello, I use WCFM marketplace, Wcfm Membership and Groups and staffs.
I have a very serious problem for several weeks on the seller registration. Indeed when a seller registers via the page “vendor-register”, after registration, instead of the thank you page, there is an error: “technical difficulties”. And this is the case even when registering by following the membership plan of the page “Vendor-membership”.
I try to set up a “Custom Thank you page”, but it does not work. I noticed that for the paid plan, after payment, the “Thank you page” works. But for the free plan the custom thank you page does not appear and the link /? Store-setup = yes returns an error
what can cause that ???
the registration of the sellers is the most important thing in a marketplace, and there it is impossible for me to continue the development of my site.
Thank you to answer me as soon as possible.
To get a more concrete idea of the problem: visit this link and try to register as a seller (there is no email verification to allow you to test quickly)
link: https://eyanoafrica.com/inscription-vendeur/