I detected this error caused by the ACS Kernel Parameter PrivacyControlEnabledP set to 1. Most likely in the 2.0.3 system this had no effect on Forums and Members portlet, but in the meantime someone added this, so that Private Data cannot be read by anyone. If someone could explain me though why forums in a .LRN instance are considered private data, this would be great. The privilege necessary is "read_private_data". I fixed it by setting the Parameter back to 0.
Therefore it seems not to be a forums privilege problem, despite the fact that I still have a lot of forums_read permissions in the system.
