Maybe I discovered some helpful information:
In the "options.txt" file, which is in my "~\AppData\Local\Ascaron Entertainment\Sacred 2" folder, there is a section called "render".
This section includes all render settings, including the key
glow = 1
Unfortunately, setting this value to 0 has no effect; here is what I have tried:
Add a line "render.glow = 0" to optionsCustom.txt
Edit the value directly in options.txt and set this file to read-only.
There is also a "video" section with some keys that could be relevant.