Unclean extusion in inner wall/internal solid infill

The inner wall of this model is always getting unclean at a certain height, which can be seen on the to surface a few layers later. I have a vcore4 idex 400mm with recommended 5.2k accel and .5 nozzle. I use Orca slicer and turned down the speed/accel and oriented them to the official profiles from RR in prusa slicer (also the PETG settings). I also tried prusa slicer, but had the same error. The flow is well calibrated and i also tuned PA. The material is PETG. I printed a small test piece, which was flawless. It printed very slow due to minimal layer time. Some parts of the larger model are also flawless, so this might have been luck. I already turned down the speed to sub 200mm/s (apart from infill). I have clean extrusion on higher speed on the outer wall. It seems like the unclean extrusion is at the internal solid infill, which is generated because of the minimum shell thickness. I also tried turning it of with the same/worse result. I initially printed with .1 layer height, but also tried with .2, with the same result.