Appointments couldn't be saved error

Hello!

First off all i want thank creators of Appointments + Plugin - i like this plugin very well.

Today i was adjusting some other settings for my clients site (staging: http://833.c4f.myftpupload.com/kontakt/) and unfortunately now i'm getting always an error that the appointment couldn't be saved. I have undone all changes made today - don't work.

Any idea?