During the SoZ/Dragon Cave quest when I enter the cave and meet Tarmas and the other guys, I can't proceed further in the cave as after the conversation if I make a few steps further, a loading screen appears. When the loading is complete, I find myself on the Overland map next to the cave entrance... I've tried several times and can not proceed deeper into the cave, this strange loading screen always pops up and puts me back to the Map and to the entrance of the cave... Moreower one of my chracater loses some HP, and if I scrool back to the messages to see what caused the damage it shows that "someone damages someone 10 (10 positive energy)".

Can someone help please? I've tried to search for such issue but no findings...

I have the COG Complete 1.23 version and using Kaedrin.

The HP loss is from Kaedrin's pack. It'll happen on every area transition and loading a saved game. I hear there's a way to disable it in a config 2DA. Check Kaedrin's readme for an option to "disable CON fix" or however it's worded.

Kaedrin's pack also changes a lot of SoZ scripts. You might want to try removing that mod entirely and see if your cave problem still happens.