I’m happy to report I was finally able to fix this problem.
Ofcourse now that I found it I feel like such a noob, but I will report it anyway in case someone is wondering:
In the event graph of my custom ABP I had a wrong event casting to the speed variable.
I changed the custom_event_1 to the Event Animate Movement and it worked right away.
Some screenshots to clarify included
Thanks again for your help ! your suggestions were pointing me in the right direction.