New entry not possible

Hi,
I’ve been using this splendid Guestbook for all my websites for years. Great plugin!

Recently, the login page always pops up once after hitting the send button for an entry. I can’t say exactly when this happened, either it was the WP update or your update. I haven’t changed anything on the site.

http://www.ferienhaus-monti-obertrubach.de/gastebuch/

I have a doubled login for security running, but I don’t understand why suddenly, its triggering the login page to appear with every entry. The check-box “only for registered user” at the backend is disabled. Any clues?

Thanks!