For those with this bug, it seems the solution is as dumb as it looks:
If you recently downgraded Skyrim using NGIO for the Grass Cache, you have to change your skyrim path again to the non stock one in MO2. It even says so in the step grass lod guide (revert changes to your skyrim).
This also explains why my xLODGEN suddenly takes 10 min to generate and not 35 min and why my Texgen refused to use HD Grass, and why Dyndolod couldn't find the appropriate version. It points to the wrong game!