Just before going my way through the mist to Vergen in chapter 2 I was at level 17, with 4 points left to achieve level 18. Into the mist I killed two enemies and got the level up message but I died afterwards (I wasn't playing with the combat mod).

When I reloaded the previous save, I couldn't achieve any single experience point, no matter what I did.

Then I installed the rebalanced combat mod, and I started gaining experience (in fact I could hear the level up sound ) but in character screen there are no talents nor level up.

Is this a common bug? Any solution to gain levels again or I'm going to make my way through the game stuck at level 17?

Often you don't get XP from killing foes in the game, or very little. It can be a little frustrating, because some of the enemies are pretty darn tough, and then you don't get any XP for them, or maybe 5. Most XP comes from completing quests. Apart from getting more monster ingredients, there doesn't appear to be much point to grind. Maybe that's a good thing, and I'm sure it's a conscious decision, but it would still be nice to get some XP after slaying a horde of harpies and so forth.

I continued playing and finished some quests. It made the level up sound and my XP growed, but no level up.

I'm playing on normal difficulty, so I think that I don't need to meditate.

Neverheless, I can manage to go through the second chapter without any problem right now, so I will keep playing and hope that it will solve itself eventually.