Problem URL return woocommerce payment and paybox e-transactions

Hello,
Sorry for my poor english.

I use wordpress (4.7.5) with plugin woocommerce (3.07) and the payment module of a bank : plugin paybox e-transactions to propose the payment with Credit Card.

The problem is that when a customer places an order it remains in failed payment under woocommerce.

I get a message from the payment module telling me this:

WARNING: Unable to join https://tourainegenealogie.org/WordPress?wc-api… for payment “M = 37 …”
Error: HTTP code: 301

It seems to be a concern at the return url, it lacks a “/” in the URL of redirection after the name “WordPress”. And yet all the other activities of the site work well. I do not know how to correct it.
For the customer happily everything works when he pays. It is only the return to the back office of woocommerce that does not happen, it is disturbing to manage the orders.

My web host and the support of the E-transactions plugin tell me that it comes from my WordPress / Woocommerce setup.

Thank you for your help.