Appointments: "Something wrong about the submitted [fieldname]"

I just installed Appointments+ and added an additional field (named 'Test'). When I do a test submit I get: "Something wrong about the submitted Test" and no data is submitted. What could be wrong?

Support access has been granted

BTW These are the short codes in the page:
[app_monthly_schedule]

[app_monthly_schedule add="1"]

TIP: Is de dag van je voorkeur al volgeboekt? Geef je dan op voor de reservelijst.

[app_pagination step="2" month="1"]

[showhide type="links" more_text="" less_text=""]

Geef je op voor de reservelijst

Als één van de afspraken vervalt dan bel ik je op.

[contact-form-7 id="433" title="Reservelijst"][/showhide]

[app_confirmation]

TIA
Michiel