Cannot transfer trips from Basecamp to RV 770

I'd like to give it another try with this issue. I did before and nothing helped.

I would like to use Basecamp 4.8.3 to create trips and then transfer these trips to my Garmin RV 770. However, it does not work. I can create the trip and seemingly send it to the RV 770 (which is connected in file transfer mode to my Mac for this operation). What is new to before is the fact that I can see the trip in Basecamp being on my RV 770 (I did not see or notice this before). However, when I then disconnect the RV 770 from the computer, shut it down, and restart it again in "normal" mode the trip is not in the Trip Planner.

What is additionally strange that the device sometimes crashes once when I try to restart it after sending a trip to it, and sometimes it does not. Furthermore, sometimes the destination of the trip appears in the saved places, and sometimes not. And especially strange is that in the very beginning I was able to transfer 4 trips to the device, but non after that.

It has been like this since I got the device, also with previous versions of Basecamp. It does not work with my two Macs - and it also does not work with a Windows 10 PC on which I installed Basecamp just for testing purposes.

When this happened in the beginning I contacted Garmin, and after some back and forth and testing and retesting they decided that something is wrong with the device and replaced it with a new device. Unfortunately it also never worked with the new devices.

Since this is not working with two devices and also not from a PC I can only assume that I do something wrong - but I have no idea, what. I am usually very good with computers; all my friends come always to me with their problems and I can usually solve them.

I am wondering whether I can get tips here to solve this. Maybe someone who knows how to do this could also hook up to my computer with Teamviewer or Splashtop, then watch me creating a trip and transferring it, and then telling me what I am doing wrong.

Thanks!