Hi Dennis,
sorry for the late reply, I was on vacation last week and only now looked at the forum.
I assume you are using the OpenFOAM6 branch of the adapter, and not any of the released archives. I have not yet (completely) ported any of the OpenFOAM.org compatibility branches to the latest adapter version with preCICE v3 support, since this is becoming more and more complicated (see and comment on Policy on support for OpenFOAM.org and other versions · Issue #328 · precice/openfoam-adapter · GitHub).
Why do you need this specific version?