I made a ranger, my favorite class in most RPG's.

In BG1 I started with about 35% stealth I think (I have 18 dexterity)
Then after a while, at level 3, I check my stats, I realize that my stealth is only of 20%

I tried unequiping all armors, weapons, etc but nothing worked.

Is that a rare bug ? I haven't found anything with google.

Although I don't know if i'm really stuck at 20, I think boots of stealth can still increase that number.

Anyone can help me ? Can it be fixed ?

No I don't have any mod installed, just the GOG install.
I guess I should have used a fix pack before launching the game though, it seems too late now.

Your problem is not the lack of a fix pack. There is something else going on. I have a completely virgin, vanilla install of BG1, and if I level a Ranger to level 3, he/she has the correct Stealth score applied. Do you have a prior save game that you could load for testing purposes? You could load it and set your Ranger's XP via the console. If he/she gets the correct stats, you know it is something that happened either during you play, or in the actual save game where the inconsistency is.

I didn't have a old save, i really should start using several save slots lol

So it took me a while cause I never used the console in BG.

But something interesting happened. I needed 9000 Xp to level up to 4 on the save that has the problem.
So I used the console to set my xp to 9000, leveled up my main and... it now displays a stealth of 53% !
I think the problem might fix itself if I continue playing then.
It might have been a "display" bug ? My actual stat was maybe still in the game somewhere...

Anyway thanks for your help ! I consider this problem fixed.
Have a nice day.