Hello! Thank you for playing :D
The ruins you're supposed to take Lucien to are actually the ones hidden beside a waterfall that's nearby, you can find them if you follow the fox in that area of the forest. Maybe I should make it more clear that that's the place he's referring to. I'm not sure why it won't let you leave the place you're at though, you should be able to return through a similar entrance as the one you entered the cave through. Sorry for the inconvenience :/
As for the glitch, I think it happens when you run in a certain direction while he's following you. I'll try and fix it on a patch soon, but for now you can try to avoid doing that.