Hi Oskar,
What version of MicroStation are you using? If you have 08.11.09.459 you can try going to the Render Settings Advanced tab and setting Irradiance Smoothing to 5 instead of 2. You will sacrifice some shadow detail but this should help. You should be using as a minimum Interior Good for your render setup and I would suggest that you increase the number of Irradiance Rays to at least 500 and add even more if necessary.
Jerry