Appointments+ :Warning: array_key_exists() ...


I have recently upgraded Appointments+ to the latest version. Now I'm having an error code on my appointments page:
Warning: array_key_exists(): The first argument should be either a string or an integer in /customers/a/7/a/ on line 725

As soon as I choose another service the message disappears. But it is back when I reload the page. I've already cleared the cache from the settings page.

Access is granted via the WPMU DEV plugin.

Can you take a look please? Thanks!