Hi @michael.komm and welcome to the FESTIM discourse!
First up, when asking a question about an error message, it is essential to provide the full error message otherwise it’s very hard for us to assist. Read before posting: How do I get my question answered?
FESTIM can “crash” in different ways.
It can be that the mesh needs to be more refined near the implantation zone, or the tolerances need to be tweaked… I can’t really say more at this stage