Film or show about Lost Civilsation OR going back in time (portal in cave?)

Dear all,
hope you can help me out, I’ve been looking for this for ages but unfortunately don’t have enough key words to find the show/movie.

When I was younger (say around 15-20 years ago), my family used to often watch a show or movie (I can’t remember which one it is). I don’t remember much with the exception of a few scenes (hopefully I’m not mixing different movies):
– a group of people travel to a hidden civilization or go back in time (or both) – a friend (researcher?) has dissapeared and they find him there (in a treehouse?)
– they encounter different groups of this civilization that rival with each other
– the group they get along with teaches them how to heal wounds with a blueish product (I think they got it from a blue flower/plant?)
– it ends with a scene in a cave where several people fight (the cave is like a portal between the worlds?), a fire spreads out, and one of the (male?) characters has to decide whether to go with his friends, or to stay with the girl he has fallen in love with. I seem to remember he is forced to go with his friends (because the fire blocks the path), but at the end he returns for her? Maybe the last part is my imagination…

God I hope I am not crazy, and this thing exist… I loved watching this.

Thank you for any help!