KeyONE BlackBerry Calendar not sending invites for non-gmail calendars

Hello everyone,

I got a new KeyONE. After a while I start to come along (used a Passport before). However, I have still one issue:
I am using a caldav calendar by a provider which is also my e-mail provider (posteo.de, so not gmail). If I add an appointment through BlackBerry Calendar, the appointment gets synced with my MacBook through caldav. That seems to work fine. However, if I add a third person as invite in the appointment, no e-mail is sent.

What I tried:
1) Setup again the caldav account. Nothing changed.
2) Using a different calendar (Google Calendar app). Also here no appointment is sent.
3) Using a GMail Calendar (not the app). Here everything works fine and invites are sent. However, I would like to use my other provider, instead of gmail for the calendar.

I am looking for a solution since days, but I cannot figure out a way to get it running.

Would be greatful if someone could help me.

Christoph