Brightness is slightly reduced in the first render after activation of Octane Server.
1. Start Blender, start Octane Server, load any project
2. Render it for the first time with data loading, save the output
3. Render it one more time or multiple times.
The very first render will be slightly less brighter than the others. After reducing GI Clamp by 0.5, the following renders look identical to the first one with reduced brightness.