Lights appear only when an object is selected

I am experimenting a strange behaviour. I created a scene and I placed in that scene a certain number of lights (I am speaking of real lights, not full-bright prims) to create a specific light & shadows effect. Yestrday night was ok. Today I took a couple of friends with me to test the scene. Since it was midday, all of us switched to midnight but... no more lights in the scene. Everything was dark. So I clicked with the right button to edit an object in the scene, and voilà... all lights appeared again. I tried several tests and the result is

1. lights do not appear in the scene
2. by keeping the RMB pressed on an object of scene, the WHOLE scene brightens

Please, note that not only the selected objects brighten, but ALL the objects, linked and NOT linked to the selected one. My friends confirmed to me that behavious, so it was not my client.

Any hint????

I've noticed that myself. Most likely culprit is that you have too many local light sources - you can only view six at a time and if you or any of your friends have face lights that could use up all of them right there.

Some tips here:
/8/e6/251701/1.html

Thank you. Only six where? In a sim? Within a specific range?

It's a client-side effect. The way it works is that you see only the six local light sources closest to you. If you have a face light or something like the light source implant from MystiTool turned on, and then you have a nearby friend or two with the same personal light sources, you might not see a light bulb on the opposite side of the room. Move closer to the light bulb and some light source where you used to be will go dim instead.

Also, while you are editing a local light source, it is automatically one of the six lights allocated to what you can see. This will appear, to you, to turn OFF one or more lights closer to you, until you let go of the more distant light source.