Well, undocumented "feature" in the LWLL addon.
So in order to get the launch & docking on FalconHeavy to work, I had to cheat a bit with the LWLL docking stuff.
IIRC, the LWLL lander vessel deletes it's docking port when attached to something... and when the Ion-6 tries to release LWLL it it calls an invalid docking port handle.
If you put the line "LAUNCH_FEATURES = false" in the LWLL cfg file that will make it work fine with Ion-6. But it will break the support for the original mission on FH...
This really should be documented in the LWLL addon, but I honestly forgot about it.