Can't add testimonials to existing group in overview

When I open a group and tick the checkboxes of the testimonials I want to add, I get a strange page when I hit the add button. I see a white screen with this text:

Thank you for creating with WordPress.

When going back to the overview page, the group is still empty.

I can of course add every testimonial by hand, but that takes a lof of time AND shouldn’t be necessary, since there is an option for it. That just doesn’t work properly.

https://wordpress.org/plugins/hms-testimonials/