You can email supportdos@larian.com, with a description of the bug, and depending on the issue, possibly the report.zip file generated by the
D:OS support tool (for the PC version). The report will contain system and game information, as well as your saved games (if the bug is system dependant or related to the saves, if it would just be illustrated by a save in the appropriate location or point in the plot). To reduce the file size, any saves not related to the issue can be deleted from the zip.
The forum is also fine, if it is something easy to reproduce.