“?” disappeared from hidden login

I’m using wps-hide login 1.61 with wordpress 5.6. On the staging site (generated with wp-staging), the login page is correctly displayed with URL/? and the new “hidden” suffix. The “?” cannot be changed in the setup with the URL/? faded in. On the production side, the “?”, which was always there, has disappeared. I neither can’t enter it as it is being cleaned from the input field and it is not shown anymore with URL faded in. The pre-faded URL, which cannot be changed, has the “?” no more. The login therefore only works if I delete the “?” by entering the Login-URL.
Since I have assigned access links with “?” to my users, they can no longer log in: What is the cause and how do I get the “?” restored?
Why does the staging site continue with the “?” and the production site not – with otherwise identical configuration?

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