How can the problem be solved?: I have 3 suggestions.
1. Introduce more areas where resting is impossible as you did with auntie Ethel's dungeon. If a player is forced to backtrack for a long time and has to invest time doing so he's less likely to just run out rest and come back. If they do so anyway consider respawning monsters into the area, reducing (or not giving) xp for their defeat.
I like restricting rests, but not in a way that causes monsters to respawn (I feel like that would potentially generate a lot of trash fights, since you won't respawn bosses/special encounters). Regardless, doing the same fights again is always boring (feels like a reload punishment).
Maybe instead, it's a set of skill-checks for you to get back to camp (depends on where you are). A combination of survival (if in nature), stealth, perception, etc. Potential random encounters due to failure. Interesting opportunities if critical success (i.e. ambush the ambushers, etc).
The only other thing we have to take into consideration is the current "teleportation" system. Maybe it's disabled within the dungeon, and a skill check is required to use (simulating a stealthy escape), or a monetary cost is required (a teleportation stone you have to buy).
2. Make time count. Again, this is already partially in the game as seen with the events of Waukeen rest. If players rest before completing a time sensitive mission consequences should be a given soing as far as causing sidequests to fail and main quest to incur into setbacks and complications.
I personally don't mind this one too (as long as it's not the entire game), but I know there are people who absolutely hate timed-scenarios. Just the idea of "time-limits" break certain people - I remember the Chapter 1 complaint threads in Kingmaker (despite the game giving you far more time than you actually will need).
I think this will work well if failure leads to an interesting scenario instead of game over - i.e. Disco Elysium
3. Make resting at camp less secure than it is now. Set a counter on the number of times certain parties rest and make those who spend way too much time at camp more likely to be attacked losing the benefits of rest altogether. Also, make resting from more dangerous zones like the putrid bog or the underdark more dangerous than doing so from a safe zone like Silvanus Grove or the Goblin Camp (as long as you are on friendly terms with the denziens, of course).
I think the idea that there's a skillcheck to get back to camp will accomplish this. This lets Larian get away with using 1x Camp location instead of having to design a new one for every new area.
