bhotting

hi, I have some trouble with my ultimaker 2. when i set the bottom layer to 0.1mm i get gaps in what should be solid. see image attached. when i set it to 0,3mm it works fine. bottom speed is 15mm/s so not vast. material is PLA Any ideas? looks like material flow is not enough or is there a known issue with cura v15.02.1 ? thanks,