Appointments+ :Warning: array_key_exists() ...

Hi,

I have recently upgraded Appointments+ to the latest version. Now I'm having an error code on my appointments page: http://www.kinesitherapiebellens.be/afspraak/
Warning: array_key_exists(): The first argument should be either a string or an integer in /customers/a/7/a/kinesitherapiebellens.be/httpd.www/wp-includes/cache.php 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!

Greetings,
Bart