Hi Kai,
We have investigated the reason why bookings are not being added to Google Calendar and the reason is pretty common. Taxibooking is trying to authenticate the Google Account with the credentials you provided, but Authentication with Google failed. This may happen for some older Google accounts. To avoid this, please follow the steps explained below.
If we use Google account original password, then in some cases, google may restrict authentication. After you try to connect by your application and failed, login to Google website by the account you want use by your application. You will see the notification the top of header of Google page(maybe you have to go to the youtube or google doc that you want to comit by app to see the notification, it says "Warning Google prevented suspicious attempt to sign in to your account. Was it you?"
Then click the link(the part of "Was it you?") and approve the sign in attempt. Google authentication will work from next requests.
OR
You can use Application Specific passwords. Please See this
tutorial to generate application specific passwords. Generate a password and use that in Google Login Password in Taxibooking Car screen.
Hope this helps.
Thanks