Wordpress Webinars Plugin: "Webinar API error: Request failed:"

[A_J]

I am receiving this error on my Wordpress page:
"Webinar API error: Request failed:"

Maybe two specialties of our setup:
I am using an internal Wordpress Server which is only accessible inhouse (https is active & working).
It's domain name is resolved by an internal DNS.

Our 3CX server is also running inhouse on another virtual machine

Do you have any ideas where to look / how to debug this behavior?

Thanks,

AJ