There are around 8 power corse on the planet.

So, no, you're not looking for nr. 4 out of 4.

EDIT: What do you mean with "Journal dissapears"?

My previous run (Light Sided) goes like that: 2 power cores from starting the quest, 1 from that droid mentioned, 1 from maintenence droid (industrial zone). Then, I've put the cores into the stations in order. Worked. Guardian talked to me, allowed me to enter M4-78 core, I did what was needed to be done there, end of the quest. Medical supplies are to be transfered to Telos, planet opened for Telos refugees, Light Sided ending. I go to Dantoine to talk with the Jedi, etc.
I've repaired droids for maintenence droid, then I was able to gain the core. Now, the problem goes as I said earlier.
Journal shows me empty space, but with glowing frame. I'll make a screenshot. EDIT: Done, they are in screenshots on my profile.
+ Kaah Ohtok disappears when I'm in the Enviromental Archon the first time. Literally it goes: I enter, battle music, Kaah's not present. When I enter later, the music is still playing. Is it because I'm Dark Sided? I've used DS responces to his speech.
It's sad, because I really enjoyed M4-78 chapter. When I updated KotOR (Steam Update), mod suddenly stoped working. I was able to move my camera and once even move around the Ebon Hawk, but when I entered planets surface game crush accured. I had only 2 mods: M4-78 and TSLRCM from moddb site. It was compatible. I deleted this mods from KotOR II folder, then uploaded this from Steam Workshop.

* The Industrial Zone core(s) are tied to the "repair parts" quest, not repairing droids.
* The screenshot implies you're using a mod that replaced/overrode M4-78EP globals.jrl
* Sounds like a mod (the same one?) overwrote our appearance.2da.

Well, only mod that I have is TSLRCM (english) from here. I've searched and deleted all components of old version mods before deleting local KotOR data throught Steam, but I'm not sure that I've deleted all. Is there some way to correct this problem?
Maintenence unit is not giving me another quest, just that with the droids and complains about maintenance imposter droid. When asked about power core, he says, that he has none for me.

Did you ask if you could help out if he mentions being low on parts?

As for your issues, best would be to uninstall KOTOR2, then go to the folder (steamapps/common/knights of the old republic II) and remove all bar saves. Then re-install. Pretty sure you don't need to re-install TSLRCM and M4-78EP due to being located differently.

He still talks about maintenece droid impostor, even when I talked with him and ended the quest. There is no other dialog option to help him.
If I delete my saves (which are on Steam Cloud), I'll still be forced to start the game from the begining? When deleting local data last time, that occured.

Remove local files shouldn't delete the savegames (they were always still there when I did that). Of course now with the workshop I never have a need to touch the main folder at all, so it wont be needed anymore.
(Could you upload your save?)

How can I do that?
+ [EDIT] Yes, I checked this. They were still there, but I had to click 'change character' button. Sorry for my stupidity :D

Worked, thanks. BTW, by accident I've found 'Want you gone' M478 version. Good work!