Server error – can't contact wp.reservationdiary.eu

I’m trying to get ReDi to work on a new site. I can reach the options tab in the plugin options, but get the message “Online booking is not available…” when saving options. The second tab (Options for Basic Package) says “Server Error in ‘/’ Application” and “Runtime error”.

I’ve been in contact with my web host and they the problem isn’t on their side but at wp.reservationdiary.eu? I’m lost here and would be so happy for help or advice!

The site is restaurang-piast.se but it’s closed and this problem is only on the back-end.

Thanks!

https://wordpress.org/plugins/redi-restaurant-reservation/