I'm playing the game with Restoration mod installed (version 1.2).

(No idea if it may influence things, but mentioning it just in case)
On Tatooine, I couldn't beat some Sand People group (in the area just outside the city, prior to their territory) so I left and did Kashyyyk part, then returned.

I got the Sand People's clothes, had everyone expect HK-47 put them on, and then went to the Enclave. I got until the entrance undisturbed, but once I entered, the guard inside turned hostile.
I do not have Zaalbar in my party, btw.
(also, after I reloaded the save just outside, one of the turrets was hostile if I recall - but it wasn't so when I first got there)

I'm not entirely sure, but I might have put the robes on after entering their area, but before moving forward.
(does it matter?)

Any ideas why it happens and what should I do?

I have a save just after leaving the city, so may re-do the whole desert (well, other than some early parts I did before leaving for Kashyyyk), but if I'm to do that, I'd rather first know what did I do wrong last time and/or how to avoid doing it now.

Yes it matters. You must put the robes on before approaching the enclave and keep them on until HK-47 has communicated with them and you've agreed to get the vaporators.

Just to be clear, by "their area" I didn't mean the enclave itself, just the area before that, with some Sand People, their banthas, mines, turrets around the enclave entrance, etc.

Anyway, thanks, I'll try that tomorrow (putting the robes on before I leave the first desert location). I've also found another hint when searching the Internet, to remove the weapons prior to approaching the turrets, that's what I suppose might also have caused the problem (if not putting the robes on immediately didn't, that is).

I vaguely remember weapons not being a problem, but might as well remove them, not like there's anything to fight there as long as the player doesn't screw up.

I haven't played this game for a long time, and I really can't remember about the weapons, but it does sound plausible. Give it a shot.

Edit: oh, and do NOT touch anything (baskets etc.) inside the enclave unless you want them to turn hostile.

Yes, I remember to load again to the level one before their main gate with the shot down ship just to put the clothes on and them not being hostile. A killer robot without camo clothes is OK though.. game logic ;)