The error-response / http-status is correct, TB asks to create the event, if it’s not already existing. As it already exists, TB get the response http-status from EGroupware which is required in that situation: 412 Precondition failed.
The problem is, TB does not expect that and does not react on this other than outputting the error. If you wait for the next periodic sync from TB, you will get a different message: The event already exists in your calendar and you can Accept or Reject the invitation.
A more sensible way for TB to react to the 412 would be to load the event from the server / EGroupware and then handle the situation, like it does, if it would have synced with the server, before the user clicked on the Invitation.
One way to deal with it on your side is to set a shorter sync interval and make your users aware of the situation.
Obviously you can also ask / sponsor TB to implement a better reaction to the 412.