i have a computer player on a map, and placed some units. I want to make all those units & buildings of that player cloaked, or at least not visible. The units stay in the same area. After a while, the units should be set to visible again.

Well, you can cloak them, but uncloaking them is not really possible from what I know...

You can replace the cloaked units with new ones. You could also reduce the amount of work to do this by placing arbiters all over then you only had to replace buildings.

What about that sprite thing that one guy posted a while ago? That would make it easier, and invisible.

Can you cloak arbiters? If so, you could just do that and move the arbiters away when you're done.
Also, I couldn't reproduce the effects(what that other guy did with the cloaking sprite).

Some people managed to reproduce them though, so it might work, but it would probably create problems, removing and such.

Only very few Toss units can be cloaked. Arbiter is unfortunately not one of them.

How about keeping a location not visible (for a while)? Even when a player moves any unit to that location, the location is still dark so that the computer player units can't be seen.

You can only do that by removing vision from the player himself and use another player that has for example map revealers to make the areas you need visible by enabling vision to this player. It would also be possible to make system where the other player gives you vision only in areas where you are. Depending on your map and amount of units you control this can get quite complex.
But be careful. If you send a not-hotkeyed unit into the black area there's no way to recover it once you deselect it.