“add auction” fails on a multi-site

I am running a multisite WordPress configuration with URL like https://<main URL>/<subdomain>.
I activated the plugin for one subdomain. I first configured the “Manage” page, then started adding an auction. When I finish this by clicking the “Add Auction” button, I am redirected to a non existing page on the main URL https://<main URL>/wp-admin/admin.php?page=wp-auctions-add.
I assume that I instead should stay in my subdomain https://<main URL>/<subdomain>/wp-admin/admin.php?page=wp-auctions-add.

When I return to the subdomain, then the auction has not been created.